MQTT Component

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A lightweight, fully-featured MQTT client implementation.

Syntax

nsoftware.IPWorksMQ.MQTT

Remarks

The MQTT component provides a lightweight, fully-featured MQTT client implementation with support for versions 3.1.1 and 5.0. The component supports plaintext and TLS-enabled connections over both standard TCP and WebSockets.

Connecting

When connecting to an MQTT server, the component sends the following information:

• The values of the ClientId, CleanSession, and KeepAliveInterval properties.
• The value of the User property (if non-empty).
• The value of the Password property (if non-empty).
• The values of the WillTopic and WillMessage properties and the WillQOS and WillRetain configuration settings.
• MQTT 5 specific values from ClientTopicAliasMax, SessionExpInterval, ConnectProperties and WillProperties.

If CleanSession is True, check the SessionPresent configuration setting once connected to determine whether the server actually had any session state saved.

Refer to CleanSession, SaveSession, and RestoreSession for more information about MQTT sessions and session state persistence; refer to WillTopic, WillMessage, WillQOS, and WillRetain for more information about MQTT Wills.

Basic Connection Example mqtt1.ClientId = "testClient"; mqtt1.CleanSession = true; mqtt1.KeepAliveInterval = 30; mqtt1.WillTopic = "wills/" + mqtt1.ClientId; mqtt1.WillMessage = mqtt1.ClientId + " was disconnected ungracefully!"; mqtt1.ConnectTo("mqtt.test-server.com", 1883);

Topic Subscriptions

When subscribing, pass one or more topic filters and QoS levels to indicate the topics to subscribe to and the desired QoS level(s). Topic filters may contain wildcards in order to match multiple topics on the server.

Subscribe Examples // Subscribed event handler. mqtt1.OnSubscribed += (s, e) => { if (e.ResponseCode <= 2) Console.WriteLine("Subscribed to " + e.TopicFilter + " at QoS " + e.QOS + "."); else Console.WriteLine("Failed to subscribe to " + e.TopicFilter + "."); }; // Basic, subscribe to some topic filters, all at the same QoS level. mqtt1.Subscribe("home,home/floor1/+/temperature,home/floor2/#", 2); // A bit more advanced, subscribe to the same topic filters, but at different QoS levels. mqtt1.Config("TopicQOSArray=1,2,2"); // The 0 is ignored here since we've specified individual QoS values explicitly. mqtt1.Subscribe("home,home/floor1/+/temperature,home/floor2/#", 0);

After subscribing to topics, any messages received will cause the MessageIn, and potentially also the MessageAck, events to fire. Refer to those events for more information about processing steps for inbound messages.

When unsubscribing, pass the exact same topic filter that was used to subscribe.

Unsubscribe Example // Unsubscribe from topic filters; have to use the exact same strings as before. If this // was to be called after calling the code example shown for the Subscribe() method, we // would still be subscribed to the "home" topic filter. mqtt1.Unsubscribe("home/floor1/+/temperature,home/floor2/#");

Refer to Subscribe and Unsubscribe for more information about subscriptions and topic filters.

Publishing Messages

PublishMessage is used to publish a message with a string payload, while PublishData is used to publish a message with a raw data payload. Both also accept the name of a topic to publish to, and a QoS level at which to publish.

Publish Examples // Publish a simple string-based message. mqtt1.PublishMessage("/home/floor1/security/camera2", 1, "Cat detected!"); // Publish a raw data message. byte[] catPicture = ...; mqtt1.PublishData("/home/floor1/security/camera2", 1, catPicture);

Refer to PublishData and PublishMessage for more information about message publishing and processing steps for outbound messages, as well as topic naming.

MQTT 5 Notes

MQTT 5 is similar to v3.1.1, with a few changes and several new features. Major differences include:

• The Clean Session flag functionality is divided into two properties to allow for finer control over session state data: the Clean Start flag (see CleanSession) and the new SessionExpInterval.
• All response packets (CONNACK, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBACK, UNSUBACK, DISCONNECT) now contain a reason code describing why operations succeeded or failed. See MessageAck, Subscribed, Unsubscribed, and DisconnectReasonCode for details.
• The Request / Response pattern is formalized by the addition of the ResponseTopic. Additional properties such as correlation data and be specified via OutgoingMessageProperties and ConnectProperties.
• New subscription features: Shared Subscriptions, Subscription Options, and SubscriptionIdentifier (see Subscribe for details).
• Topic Aliases can be sent by both client and server to refer to topic filters by shorter numerical identifiers in order to save bandwidth (see TopicAlias, ServerTopicAliasMax and ClientTopicAliasMax).
• Servers can communicate what features it supports in ConnAckProperties.
• More: message expiration (OutgoingMessageProperties), Receive Maximums and Maximum Packet Sizes (ConnectProperties and ConnAckProperties), and a Will Delay interval (WillProperties) are all supported.

Property List

The following is the full list of the properties of the component with short descriptions. Click on the links for further details.

Method List

The following is the full list of the methods of the component with short descriptions. Click on the links for further details.

Event List

The following is the full list of the events fired by the component with short descriptions. Click on the links for further details.

Config Settings

The following is a list of config settings for the component with short descriptions. Click on the links for further details.

CleanSession Property (MQTT Component)

Determines whether a clean session is used once connected.

Syntax

• C#
• VB.NET

public bool CleanSession { get; set; }

Public Property CleanSession As Boolean

Default Value

True

Remarks

This property determines whether or not the component should instruct the server to use a clean session when it connects. (Note that this property must be set to the desired value before calling Connect.)

By default, CleanSession is true, so the server will discard any state data previously associated with the current ClientId once the component has connected successfully. In addition, the server will not save any state data when the component disconnects.

Setting CleanSession to False before connecting will cause the server to re-associate any previously stored state data for the current ClientId. The server will also save any state data that exists when the component is disconnected.

The server-side session state consists of:

• whether a session is present (see SessionPresent),
• client subscriptions (including subscription identifiers in MQTT 5),
• QoS 1 and 2 messages sent to the client pending transmission or completed acknowledgment (and optionally pending QoS 0 messages),
• QoS 2 messages received from the client but not completely acknowledged, and
• the SessionExpInterval (in MQTT 5).

Note that retained messages are not deleted as a result of a session ending, but are not part of the session state.

Client-Side Session State

When setting CleanSession to False, it is important that the RestoreSession method be used to load any previously saved (client-side) session data from a previous session before calling Connect. (Successful calls to RestoreSession will automatically set CleanSession to False.)

Similarly, once disconnected, save the string returned by SaveSession for later use. The saved session information may be used when connecting with CleanSession set to False at a later time.

The client-side session state consists of:

• QoS 1 and QoS 2 messages which have been sent to the Server, but have not been completely acknowledged, and
• QoS 2 messages which have been received from the Server, but have not been completely acknowledged.

Refer to SaveSession and RestoreSession for more information.

MQTT 5 Notes

In MQTT 5, the "Clean Session" flag from v3.1.1 is split into a "Clean Start" flag indicating that the server should discard previously saved session data from this connection, and a SessionExpInterval controlling the length of time the server should preserve the session data after disconnection.

In MQTT 5, this property acts only as the "Clean Start" flag. It determines whether or not the component should instruct the server to discard any previously stored session data associated with the current ClientId and start a new clean session.

For more information on session expiration in MQTT 5, see the SessionExpInterval config. Setting CleanSession to True in v3.1.1 is equivalent to setting CleanSession to True AND setting SessionExpInterval to 0 in MQTT 5.

As with MQTT 3.1.1, it is important for the client to save and restore its own session data on connection and disconnection depending on the value of CleanSession, and to manage its preservation depending on SessionExpInterval.

This property is not available at design time.

ClientId Property (MQTT Component)

A string that uniquely identifies this instance of the component to the server.

Syntax

• C#
• VB.NET

public string ClientId { get; set; }

Public Property ClientId As String

Default Value

""

Remarks

The ClientId string is used by the server to uniquely identify each client that is connected to it. In MQTT 5, when the SessionExpInterval config is set to a positive value, the server will also associate it with any session state data that needs to be saved for the length of time specified when the component is disconnected.

If ClientId is empty when Connect is called, the component's behavior depends on value of CleanSession. If CleanSession is True, the component will automatically generate a unique value for ClientId before connecting. If CleanSession is False, the component throws an exception.

This property is not available at design time.

Connected Property (MQTT Component)

Triggers a connection or disconnection.

Syntax

• C#
• VB.NET

public bool Connected { get; set; }

Public Property Connected As Boolean

Default Value

False

Remarks

This property triggers a connection or disconnection. Setting this property to True makes the component attempt to connect to the host identified by the RemoteHost property. If successful, after the connection is achieved the value of the property changes to True and the Connected event is fired.

Setting this property to False closes the connection.

By default the component will connect in plaintext. To enable SSL set SSLEnabled to True.

In addition, WebSocket connections are supported. To connect using WebSockets specify a hostname beginning with ws:// (plaintext) or wss:// (SSL). For instance, ws://test.mosquitto.org.

When connecting to an MQTT server, the component sends the following information:

• The values of the ClientId, CleanSession, and KeepAliveInterval properties.
• The value of the User property (if non-empty).
• The value of the Password property (if non-empty).
• The values of the WillTopic and WillMessage properties and the WillQOS and WillRetain configuration settings.
• MQTT 5 specific values from ClientTopicAliasMax, SessionExpInterval, ConnectProperties and WillProperties.

If CleanSession is True, check the SessionPresent configuration setting once connected to determine whether the server actually had any session state saved.

Refer to CleanSession, SaveSession, and RestoreSession for more information about MQTT sessions and session state persistence; refer to WillTopic, WillMessage, WillQOS, and WillRetain for more information about MQTT Wills.

Basic Connection Example mqtt1.ClientId = "testClient"; mqtt1.CleanSession = true; mqtt1.KeepAliveInterval = 30; mqtt1.WillTopic = "wills/" + mqtt1.ClientId; mqtt1.WillMessage = mqtt1.ClientId + " was disconnected ungracefully!"; mqtt1.RemoteHost = "mqtt.test-server.com"; mqtt1.RemotePort = 1883; mqtt1.Connect();

This property is not available at design time.

Firewall Property (MQTT Component)

A set of properties related to firewall access.

Syntax

• C#
• VB.NET

public Firewall Firewall { get; set; }

Public Property Firewall As Firewall

Remarks

This is a Firewall-type property, which contains fields describing the firewall through which the component will attempt to connect.

IncomingMessages Property (MQTT Component)

Collection of incoming messages with QoS > 0 that have not been fully acknowledged.

Syntax

• C#
• VB.NET

public MQTTMessageList IncomingMessages { get; }

Public ReadOnly Property IncomingMessages As MQTTMessageList

Remarks

Each time the component receives a message with a QoS of 1 or 2, it adds it to the IncomingMessages collection. A message remains in the IncomingMessages collection until the component has completed all of the acknowledgment steps required for it based on its QoS level.

Incoming messages with a QoS of 1 follow these steps:

1. The message is added to IncomingMessages when the component receives the PUBLISH packet.
2. The component sends a PUBACK (publish acknowledgment) packet in response.
3. The MessageAck event is fired.
4. The message is removed from IncomingMessages.
5. The MessageIn event is fired.

Incoming messages with a QoS of 2 follow these steps:

1. The message is added to IncomingMessages when the component receives the PUBLISH packet.
2. The component sends a PUBREC (publish received) packet in response.
3. The component waits to receive a PUBREL (publish release) packet.
4. The component sends a PUBCOMP (publish complete) packet in response.
5. The MessageAck event is fired.
6. The message is removed from IncomingMessages.
7. The MessageIn event is fired.

The component processes messages anytime DoEvents is called. In applications with a GUI, the DoEvents method is also typically called automatically when the component is idle. In all other situations however, it is best practice to call DoEvents in a loop when possible to ensure timely processing of messages.

Note that incoming messages with a QoS of 0 are not added to IncomingMessages since they do not require acknowledgment. Upon receipt, they are added to an internal queue, and are fired through the MessageIn event as soon as DoEvents is called.

This collection is indexed from 0 to count -1.

This property is read-only and not available at design time.

KeepAliveInterval Property (MQTT Component)

The maximum period of inactivity the component will allow before sending a keep-alive packet.

Syntax

• C#
• VB.NET

public int KeepAliveInterval { get; set; }

Public Property KeepAliveInterval As Integer

Default Value

0

Remarks

The KeepAliveInterval, if set to a non-zero value, is the maximum number of seconds that the component will allow the connection to be idle before sending a PINGREQ (ping request) packet to the server. The value of KeepAliveInterval is sent to the server when Connect is called; it cannot be changed when the component is already connected.

MQTT servers are required to measure periods of inactivity for all clients who specify a non-zero KeepAliveInterval, and must disconnect them if they have not communicated within 1.5 times KeepAliveInterval.

Similarly, if the component doesn't receive a PINGRESP (ping response) packet within KeepAliveInterval seconds after sending a PINGREQ packet, it will disconnect from the server.

If KeepAliveInterval is set to 0 (default), keep-alive functionality is disabled entirely. In this case, the component will not send any PINGREQ packets, and the server is not required to close the connection due to inactivity.

Note that, regardless of keep-alive settings, the server is always free to disconnect clients it deems "unresponsive".

This property is not available at design time.

LocalHost Property (MQTT Component)

The name of the local host or user-assigned IP interface through which connections are initiated or accepted.

Syntax

• C#
• VB.NET

public string LocalHost { get; set; }

Public Property LocalHost As String

Default Value

""

Remarks

This property contains the name of the local host as obtained by the gethostname() system call, or if the user has assigned an IP address, the value of that address.

In multihomed hosts (machines with more than one IP interface) setting LocalHost to the IP address of an interface will make the component initiate connections (or accept in the case of server components) only through that interface. It is recommended to provide an IP address rather than a hostname when setting this property to ensure the desired interface is used.

If the component is connected, the LocalHost property shows the IP address of the interface through which the connection is made in internet dotted format (aaa.bbb.ccc.ddd). In most cases, this is the address of the local host, except for multihomed hosts (machines with more than one IP interface).

Note: LocalHost is not persistent. You must always set it in code, and never in the property window.

LocalPort Property (MQTT Component)

The TCP port in the local host where the component binds.

Syntax

• C#
• VB.NET

public int LocalPort { get; set; }

Public Property LocalPort As Integer

Default Value

0

Remarks

This property must be set before a connection is attempted. It instructs the component to bind to a specific port (or communication endpoint) in the local machine.

Setting this property to 0 (default) enables the system to choose an open port at random. The chosen port will be returned by the LocalPort property after the connection is established.

LocalPort cannot be changed once a connection is made. Any attempt to set this property when a connection is active will generate an error.

This property is useful when trying to connect to services that require a trusted port on the client side.

OutgoingMessages Property (MQTT Component)

Collection of outgoing messages with QoS > 0 that have not been fully acknowledged.

Syntax

• C#
• VB.NET

public MQTTMessageList OutgoingMessages { get; }

Public ReadOnly Property OutgoingMessages As MQTTMessageList

Remarks

Each time the component publishes a message with a QoS of 1 or 2, it adds it to the OutgoingMessages collection after the initial PUBLISH packet is sent (this does not occur for messages publish with a QoS of 0). A message remains in the OutgoingMessages collection until the component has completed all of the acknowledgment steps required for it based on its QoS level.

Outgoing messages with a QoS of 1 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBACK (publish acknowledgment) packet.
3. The MessageAck event is fired.
4. The message is removed from OutgoingMessages.
5. The MessageOut event is fired.

Outgoing messages with a QoS of 2 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBREC (publish received) packet.
3. The component sends a PUBREL (publish release) packet in response.
4. The component waits to receive a PUBCOMP (publish complete) packet.
5. The MessageAck event is fired.
6. The message is removed from OutgoingMessages.
7. The MessageOut event is fired.

In MQTT 3.1.1, the RepublishInterval configuration setting, if set to a non-zero value (default), controls how long the component will wait to receive a PUBACK (for QoS 1) or PUBREC (for QoS 2) before automatically republishing an outgoing message. In MQTT 5, messages are only republished if the client is disconnected before receiving a PUBACK or PUBREC.

The component processes messages anytime DoEvents is called. In applications with a GUI, the DoEvents method is also typically called automatically when the component is idle. In all other situations however, it is best practice to call DoEvents in a loop when possible to ensure timely processing of messages.

This collection is indexed from 0 to count -1.

This property is read-only and not available at design time.

Password Property (MQTT Component)

A password if authentication is to be used.

Syntax

• C#
• VB.NET

public string Password { get; set; }

Public Property Password As String

Default Value

""

Remarks

This property can be set to a password if authentication is to be used.

In MQTT 3.1.1, while a User may be specified without a Password, setting a Password without a User is not supported; attempting to do so will cause the server to reject the connection attempt.

In MQTT 5, a Password may be set without a User, allowing the field to be used for credentials other than a password.

This property is not available at design time.

ReadyToSend Property (MQTT Component)

Indicates whether the component is ready to send data.

Syntax

• C#
• VB.NET

public bool ReadyToSend { get; }

Public ReadOnly Property ReadyToSend As Boolean

Default Value

False

Remarks

This property indicates that the underlying TCP/IP subsystem is ready to accept data. This is True after connecting to the remote host, and will become False if a call to PublishData or PublishMessage fails due to a WOULDBLOCK condition.

Once data can be sent again, the ReadyToSend event will fire and this property will be True.

If a WOULDBLOCK error occurs while sending a message the component will automatically complete the transmission when sending is possible. No action needs to be taken to re-send the message. ReadyToSend will fire after the component completes any partially sent messages.

This property is read-only and not available at design time.

RemoteHost Property (MQTT Component)

The address of the remote host. Domain names are resolved to IP addresses.

Syntax

• C#
• VB.NET

public string RemoteHost { get; set; }

Public Property RemoteHost As String

Default Value

""

Remarks

This property specifies the IP address (IP number in dotted internet format) or Domain Name of the remote host. It is set before a connection is attempted and cannot be changed once a connection is established.

If this property is set to a Domain Name, a DNS request is initiated, and upon successful termination of the request, this property is set to the corresponding address. If the search is not successful, an error is returned.

If the component is configured to use a SOCKS firewall, the value assigned to this property may be preceded with an "*". If this is the case, the host name is passed to the firewall unresolved and the firewall performs the DNS resolution.

By default the component will connect in plaintext. To enable SSL set SSLEnabled to True.

In addition, WebSocket connections are supported. To connect using WebSockets specify a hostname beginning with ws:// (plaintext) or wss:// (SSL). For instance, ws://test.mosquitto.org.

RemotePort Property (MQTT Component)

The port of the MQTT server (default is 1883). The default port for SSL is 8883.

Syntax

• C#
• VB.NET

public int RemotePort { get; set; }

Public Property RemotePort As Integer

Default Value

1883

Remarks

This property specifies a service port on the remote host to connect to.

A valid port number (a value between 1 and 65535) is required for the connection to take place. The property must be set before a connection is attempted and cannot be changed once a connection is established. Any attempt to change this property while connected will fail with an error.

This property is not available at design time.

SSLAcceptServerCert Property (MQTT Component)

Instructs the component to unconditionally accept the server certificate that matches the supplied certificate.

Syntax

• C#
• VB.NET

public Certificate SSLAcceptServerCert { get; set; }

Public Property SSLAcceptServerCert As Certificate

Remarks

If it finds any issues with the certificate presented by the server, the component will normally terminate the connection with an error.

You may override this behavior by supplying a value for SSLAcceptServerCert. If the certificate supplied in SSLAcceptServerCert is the same as the certificate presented by the server, then the server certificate is accepted unconditionally, and the connection will continue normally.

Note: This functionality is provided only for cases in which you otherwise know that you are communicating with the right server. If used improperly, this property may create a security breach. Use it at your own risk.

SSLCert Property (MQTT Component)

The certificate to be used during Secure Sockets Layer (SSL) negotiation.

Syntax

• C#
• VB.NET

public Certificate SSLCert { get; set; }

Public Property SSLCert As Certificate

Remarks

This property includes the digital certificate that the component will use during SSL negotiation. Set this property to a valid certificate before starting SSL negotiation. To set a certificate, you may set the Encoded field to the encoded certificate. To select a certificate, use the store and subject fields.

SSLEnabled Property (MQTT Component)

This property indicates whether Transport Layer Security/Secure Sockets Layer (TLS/SSL) is enabled.

Syntax

• C#
• VB.NET

public bool SSLEnabled { get; set; }

Public Property SSLEnabled As Boolean

Default Value

False

Remarks

This property specifies whether TLS/SSL is enabled in the component. When False (default), the component operates in plaintext mode. When True, TLS/SSL is enabled.

TLS/SSL may also be enabled by setting SSLStartMode. Setting SSLStartMode will automatically update this property value.

This property is not available at design time.

SSLProvider Property (MQTT Component)

The Secure Sockets Layer/Transport Layer Security (SSL/TLS) implementation to use.

Syntax

• C#
• VB.NET

public MQTTSSLProviders SSLProvider { get; set; }

enum MQTTSSLProviders {
  sslpAutomatic,
  sslpPlatform,
  sslpInternal
}

Public Property SSLProvider As MqttSSLProviders

Enum MQTTSSLProviders
  sslpAutomatic
  sslpPlatform
  sslpInternal
End Enum

Default Value

0

Remarks

This property specifies the SSL/TLS implementation to use. In most cases the default value of 0 (Automatic) is recommended and should not be changed. When set to 0 (Automatic), the component will select whether to use the platform implementation or the internal implementation depending on the operating system as well as the TLS version being used.

Possible values are as follows:

In most cases using the default value (Automatic) is recommended. The component will select a provider depending on the current platform.

When Automatic is selected, on Windows, the component will use the platform implementation. On Linux/macOS, the component will use the internal implementation. When TLS 1.3 is enabled via SSLEnabledProtocols, the internal implementation is used on all platforms.

The .NET Standard library will always use the internal implementation on all platforms.

SSLServerCert Property (MQTT Component)

The server certificate for the last established connection.

Syntax

• C#
• VB.NET

public Certificate SSLServerCert { get; }

Public ReadOnly Property SSLServerCert As Certificate

Remarks

This property contains the server certificate for the last established connection.

SSLServerCert is reset every time a new connection is attempted.

This property is read-only.

Timeout Property (MQTT Component)

This property includes the timeout for the component.

Syntax

• C#
• VB.NET

public int Timeout { get; set; }

Public Property Timeout As Integer

Default Value

60

Remarks

This property defines the timeout when sending data. When SSLEnabled is False a value of 0 means data will be sent asynchronously and a positive value means data is sent synchronously. When SSLEnabled is True all data is sent synchronously regardless of the Timeout value. Please see the notes below for details.

Plaintext

If the Timeout property is set to 0, all operations return immediately, potentially failing with a WOULDBLOCK error if data cannot be sent immediately.

If Timeout is set to a positive value, data is sent in a blocking manner and the component will wait for the operation to complete before returning control. The component will handle any potential WOULDBLOCK errors internally and automatically retry the operation for a maximum of Timeout seconds.

SSL

If the Timeout property is set to 0, all operations will run uninterrupted until successful completion or an error condition is encountered.

If Timeout is set to a positive value, the component will wait for the operation to complete before returning control.

Additional Notes

The component will use DoEvents to enter an efficient wait loop during any potential waiting period, making sure that all system events are processed immediately as they arrive. This ensures that the host application does not freeze and remains responsive.

If Timeout expires, and the operation is not yet complete, the component throws an exception.

Note: By default, all timeouts are inactivity timeouts, that is, the timeout period is extended by Timeout seconds when any amount of data is successfully sent or received.

The default value for the Timeout property is 60 seconds.

User Property (MQTT Component)

A username if authentication is to be used.

Syntax

• C#
• VB.NET

public string User { get; set; }

Public Property User As String

Default Value

""

Remarks

This property can be set to a username if authentication is to be used.

In MQTT 3.1.1, while a User may be specified without a Password, setting a Password without a User is not supported; attempting to do so will cause the server to reject the connection attempt.

In MQTT 5, a Password may be set without a User, allowing the field to be used for credentials other than a password.

This property is not available at design time.

Version Property (MQTT Component)

The MQTT protocol version that the component will conform to.

Syntax

• C#
• VB.NET

public MQTTVersions Version { get; set; }

enum MQTTVersions {
  mvV3,
  mvV5
}

Public Property Version As MqttVersions

Enum MQTTVersions
  mvV3
  mvV5
End Enum

Default Value

0

Remarks

This property specifies the version of the MQTT protocol to use. Possible values are:

This property is not available at design time.

WillMessage Property (MQTT Component)

The message that the server should publish in the event of an ungraceful disconnection.

Syntax

• C#
• VB.NET

public string WillMessage { get; set; }

Public Property WillMessage As String

Default Value

""

Remarks

This property may be set before calling Connect to specify to the server a message that should be published on WillTopic if the connection is closed ungracefully. Since it is sent to the server when Connect is called, this property's value cannot be changed when already connected.

Note that the WillMessage will only be sent to the server when Connect is called if WillTopic is set.

Refer to WillTopic for more information about MQTT Will functionality.

This property is not available at design time.

WillTopic Property (MQTT Component)

The topic that the server should publish the WillMessage to in the event of an ungraceful disconnection.

Syntax

• C#
• VB.NET

public string WillTopic { get; set; }

Public Property WillTopic As String

Default Value

""

Remarks

This property may be set before calling Connect to specify the topic name that the server should publish the WillMessage on if the connection is closed ungracefully. Since it is sent to the server when Connect is called, this property's value cannot be changed when already connected.

MQTT Wills

The Will feature of MQTT allows a client to specify to the server a WillMessage to publish (as well as a WillTopic to publish it on) in the event of an ungraceful disconnection.

An "ungraceful disconnection" is any disconnection other than one triggered by calling Disconnect (in which case the server discards the Will message without publishing it). Note that in MQTT 5, the client can set a DisconnectReasonCode of 0x04 before calling Disconnect to instruct the server to publish the Will message anyway.

In addition to the WillTopic and WillMessage properties, the WillQOS setting may be used to specify the Will message's QoS level, and the WillRetain setting to set the Will message's Retain flag. Refer to those settings for more information.

If WillTopic is set to empty string (default) when Connect is called, the component will not send a Will to the server.

In MQTT 5, the "WillDelayInterval" value in the WillProperties config can specify a delay between the ending of the connection and sending the will message, so that the WillMessage will not be sent if a connection is re-established within a certain period of time.

This property is not available at design time.

Config Method (MQTT Component)

Sets or retrieves a configuration setting.

Syntax

• C#
• VB.NET

public string Config(string configurationString);

Async Version
public async Task<string> Config(string configurationString);
public async Task<string> Config(string configurationString, CancellationToken cancellationToken);

Public Function Config(ByVal ConfigurationString As String) As String

Async Version
Public Function Config(ByVal ConfigurationString As String) As Task(Of String)
Public Function Config(ByVal ConfigurationString As String, cancellationToken As CancellationToken) As Task(Of String)

Remarks

Config is a generic method available in every component. It is used to set and retrieve configuration settings for the component.

These settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the component, access to these internal properties is provided through the Config method.

To set a configuration setting named PROPERTY, you must call Config("PROPERTY=VALUE"), where VALUE is the value of the setting expressed as a string. For boolean values, use the strings "True", "False", "0", "1", "Yes", or "No" (case does not matter).

To read (query) the value of a configuration setting, you must call Config("PROPERTY"). The value will be returned as a string.

Connect Method (MQTT Component)

Connects to the remote host.

Syntax

• C#
• VB.NET

public void Connect();

Async Version
public async Task Connect();
public async Task Connect(CancellationToken cancellationToken);

Public Sub Connect()

Async Version
Public Sub Connect() As Task
Public Sub Connect(cancellationToken As CancellationToken) As Task

Remarks

This method connects to the remote host specified by RemoteHost and RemotePort. Calling this method is equivalent to setting the Connected property to True.

By default the component will connect in plaintext. To enable SSL set SSLEnabled to True.

In addition, WebSocket connections are supported. To connect using WebSockets specify a hostname beginning with ws:// (plaintext) or wss:// (SSL). For instance, ws://test.mosquitto.org.

When connecting to an MQTT server, the component sends the following information:

• The values of the ClientId, CleanSession, and KeepAliveInterval properties.
• The value of the User property (if non-empty).
• The value of the Password property (if non-empty).
• The values of the WillTopic and WillMessage properties and the WillQOS and WillRetain configuration settings.
• MQTT 5 specific values from ClientTopicAliasMax, SessionExpInterval, ConnectProperties and WillProperties.

If CleanSession is True, check the SessionPresent configuration setting once connected to determine whether the server actually had any session state saved.

Refer to CleanSession, SaveSession, and RestoreSession for more information about MQTT sessions and session state persistence; refer to WillTopic, WillMessage, WillQOS, and WillRetain for more information about MQTT Wills.

Basic Connection Example mqtt1.ClientId = "testClient"; mqtt1.CleanSession = true; mqtt1.KeepAliveInterval = 30; mqtt1.WillTopic = "wills/" + mqtt1.ClientId; mqtt1.WillMessage = mqtt1.ClientId + " was disconnected ungracefully!"; mqtt1.RemoteHost = "mqtt.test-server.com"; mqtt1.RemotePort = 1883; mqtt1.Connect();

ConnectTo Method (MQTT Component)

Connects to the remote host.

Syntax

• C#
• VB.NET

public void ConnectTo(string host, int port);

Async Version
public async Task ConnectTo(string host, int port);
public async Task ConnectTo(string host, int port, CancellationToken cancellationToken);

Public Sub ConnectTo(ByVal Host As String, ByVal Port As Integer)

Async Version
Public Sub ConnectTo(ByVal Host As String, ByVal Port As Integer) As Task
Public Sub ConnectTo(ByVal Host As String, ByVal Port As Integer, cancellationToken As CancellationToken) As Task

Remarks

This method connects to the remote host specified by the Host and Port parameters. Calling this method is equivalent to setting the RemoteHost property to Host, setting RemotePort to Port, and then setting the Connected property to True.

By default the component will connect in plaintext. To enable SSL set SSLEnabled to True.

In addition, WebSocket connections are supported. To connect using WebSockets specify a hostname beginning with ws:// (plaintext) or wss:// (SSL). For instance, ws://test.mosquitto.org.

When connecting to an MQTT server, the component sends the following information:

• The values of the ClientId, CleanSession, and KeepAliveInterval properties.
• The value of the User property (if non-empty).
• The value of the Password property (if non-empty).
• The values of the WillTopic and WillMessage properties and the WillQOS and WillRetain configuration settings.
• MQTT 5 specific values from ClientTopicAliasMax, SessionExpInterval, ConnectProperties and WillProperties.

If CleanSession is True, check the SessionPresent configuration setting once connected to determine whether the server actually had any session state saved.

Refer to CleanSession, SaveSession, and RestoreSession for more information about MQTT sessions and session state persistence; refer to WillTopic, WillMessage, WillQOS, and WillRetain for more information about MQTT Wills.

Basic Connection Example mqtt1.ClientId = "testClient"; mqtt1.CleanSession = true; mqtt1.KeepAliveInterval = 30; mqtt1.WillTopic = "wills/" + mqtt1.ClientId; mqtt1.WillMessage = mqtt1.ClientId + " was disconnected ungracefully!"; mqtt1.ConnectTo("mqtt.test-server.com", 1883);

Disconnect Method (MQTT Component)

This method disconnects from the remote host.

Syntax

• C#
• VB.NET

public void Disconnect();

Async Version
public async Task Disconnect();
public async Task Disconnect(CancellationToken cancellationToken);

Public Sub Disconnect()

Async Version
Public Sub Disconnect() As Task
Public Sub Disconnect(cancellationToken As CancellationToken) As Task

Remarks

This method disconnects from the remote host. Calling this method is equivalent to setting the Connected property to False.

Refer to the Connected property for more information about MQTT-specific behavior.

DoEvents Method (MQTT Component)

This method processes events from the internal message queue.

Syntax

• C#
• VB.NET

public void DoEvents();

Async Version
public async Task DoEvents();
public async Task DoEvents(CancellationToken cancellationToken);

Public Sub DoEvents()

Async Version
Public Sub DoEvents() As Task
Public Sub DoEvents(cancellationToken As CancellationToken) As Task

Remarks

When DoEvents is called, the component processes any available events. If no events are available, it waits for a preset period of time, and then returns.

Interrupt Method (MQTT Component)

Interrupt the current action and disconnects from the remote host.

Syntax

• C#
• VB.NET

public void Interrupt();

Async Version
public async Task Interrupt();
public async Task Interrupt(CancellationToken cancellationToken);

Public Sub Interrupt()

Async Version
Public Sub Interrupt() As Task
Public Sub Interrupt(cancellationToken As CancellationToken) As Task

Remarks

This method will interrupt the current method (if applicable) and cause the component to disconnect from the remote host.

PublishData Method (MQTT Component)

Publishes a message with a raw data payload.

Syntax

• C#
• VB.NET

public void PublishData(string topic, int QOS, byte[] data);

Async Version
public async Task PublishData(string topic, int QOS, byte[] data);
public async Task PublishData(string topic, int QOS, byte[] data, CancellationToken cancellationToken);

Public Sub PublishData(ByVal Topic As String, ByVal QOS As Integer, ByVal Data As String)

Async Version
Public Sub PublishData(ByVal Topic As String, ByVal QOS As Integer, ByVal Data As String) As Task
Public Sub PublishData(ByVal Topic As String, ByVal QOS As Integer, ByVal Data As String, cancellationToken As CancellationToken) As Task

Remarks

This method publishes an MQTT message with a raw data payload to the specified Topic at a given QOS level.

Publish Examples // Publish a simple string-based message. mqtt1.PublishMessage("/home/floor1/security/camera2", 1, "Cat detected!"); // Publish a raw data message. byte[] catPicture = ...; mqtt1.PublishData("/home/floor1/security/camera2", 1, catPicture);

The Retain configuration setting may be set before calling this method in order to publish a retained message (see Retain for more information).

Topic Names

Topic names are case-sensitive, must be 1-65535 characters long, and may include any characters except wildcard characters (# and +) and the null character. The / character separates levels within a topic name, which is important in the context of subscribing (see Subscribe for more information).

Keep in mind that using topic names with leading or trailing / characters will cause topic levels with zero-length names to be created. That is, a topic name like /a/b/ consists of the levels '', 'a', 'b', and ''. Depending on the server, multiple successive /s may also cause zero-length levels to be created, or may be treated as a single /.

Topic names that begin with a $ are "system topics", and servers will typically prevent clients from publishing to them.

For example, topic names that begin with $share are reserved for Shared Subscriptions (see Subscribe).

QoS Values

QoS values set the service level for delivery of a message. Values range from 0 to 2 and have the following meanings:

QoS is not guaranteed to be end-to-end in MQTT. The server must downgrade a message's QoS level when delivering it to clients who specified a lower "maximum acceptable" QoS when they subscribed. For example, if Client X subscribes to a topic at QoS 1, and Client Y publishes a message to that topic at QoS 2, the server will downgrade the message to QoS 1 when attempting to deliver it to Client X.

Outbound Message Processing

Outgoing messages with a QoS of 1 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBACK (publish acknowledgment) packet.
3. The MessageAck event is fired.
4. The message is removed from OutgoingMessages.
5. The MessageOut event is fired.

Outgoing messages with a QoS of 2 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBREC (publish received) packet.
3. The component sends a PUBREL (publish release) packet in response.
4. The component waits to receive a PUBCOMP (publish complete) packet.
5. The MessageAck event is fired.
6. The message is removed from OutgoingMessages.
7. The MessageOut event is fired.

In MQTT 3.1.1, the RepublishInterval configuration setting, if set to a non-zero value (default), controls how long the component will wait to receive a PUBACK (for QoS 1) or PUBREC (for QoS 2) before automatically republishing an outgoing message. In MQTT 5, messages are only republished if the client is disconnected before receiving a PUBACK or PUBREC.

MQTT 5 Notes

MQTT 5 makes a number of new features available when publishing messages, including:

• TopicAlias - Set an integer alias along with the topic name when first publishing a message. When publishing additional messages to the same topic if TopicAlias is set the Topic parameter does not need to be set. This reduces the size of the message sent to the server.
• Request / Response - Set a ResponseTopic which identifies the topic to which a receiver should respond.

Some of these features require configuration settings to be set prior to publishing. In these cases, note that these configuration settings will continue to be applicable to any future PUBLISH packets sent with the method unless they are updated or reset. To prevent clear a previously set configuration setting, set it to "", or -1 (for integers).

See the OutgoingMessageProperties configuration setting for additional details.

PublishMessage Method (MQTT Component)

Publishes a message with a string payload.

Syntax

• C#
• VB.NET

public void PublishMessage(string topic, int QOS, string message);

Async Version
public async Task PublishMessage(string topic, int QOS, string message);
public async Task PublishMessage(string topic, int QOS, string message, CancellationToken cancellationToken);

Public Sub PublishMessage(ByVal Topic As String, ByVal QOS As Integer, ByVal Message As String)

Async Version
Public Sub PublishMessage(ByVal Topic As String, ByVal QOS As Integer, ByVal Message As String) As Task
Public Sub PublishMessage(ByVal Topic As String, ByVal QOS As Integer, ByVal Message As String, cancellationToken As CancellationToken) As Task

Remarks

This method publishes an MQTT message with a string payload to the specified Topic at a given QOS level.

Publish Examples // Publish a simple string-based message. mqtt1.PublishMessage("/home/floor1/security/camera2", 1, "Cat detected!"); // Publish a raw data message. byte[] catPicture = ...; mqtt1.PublishData("/home/floor1/security/camera2", 1, catPicture);

The Retain configuration setting may be set before calling this method in order to publish a retained message (see Retain for more information).

Topic Names

Topic names are case-sensitive, must be 1-65535 characters long, and may include any characters except wildcard characters (# and +) and the null character. The / character separates levels within a topic name, which is important in the context of subscribing (see Subscribe for more information).

Keep in mind that using topic names with leading or trailing / characters will cause topic levels with zero-length names to be created. That is, a topic name like /a/b/ consists of the levels '', 'a', 'b', and ''. Depending on the server, multiple successive /s may also cause zero-length levels to be created, or may be treated as a single /.

Topic names that begin with a $ are "system topics", and servers will typically prevent clients from publishing to them.

For example, topic names that begin with $share are reserved for Shared Subscriptions (see Subscribe).

QoS Values

QoS values set the service level for delivery of a message. Values range from 0 to 2 and have the following meanings:

QoS is not guaranteed to be end-to-end in MQTT. The server must downgrade a message's QoS level when delivering it to clients who specified a lower "maximum acceptable" QoS when they subscribed. For example, if Client X subscribes to a topic at QoS 1, and Client Y publishes a message to that topic at QoS 2, the server will downgrade the message to QoS 1 when attempting to deliver it to Client X.

Outbound Message Processing

Outgoing messages with a QoS of 1 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBACK (publish acknowledgment) packet.
3. The MessageAck event is fired.
4. The message is removed from OutgoingMessages.
5. The MessageOut event is fired.

Outgoing messages with a QoS of 2 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBREC (publish received) packet.
3. The component sends a PUBREL (publish release) packet in response.
4. The component waits to receive a PUBCOMP (publish complete) packet.
5. The MessageAck event is fired.
6. The message is removed from OutgoingMessages.
7. The MessageOut event is fired.

In MQTT 3.1.1, the RepublishInterval configuration setting, if set to a non-zero value (default), controls how long the component will wait to receive a PUBACK (for QoS 1) or PUBREC (for QoS 2) before automatically republishing an outgoing message. In MQTT 5, messages are only republished if the client is disconnected before receiving a PUBACK or PUBREC.

MQTT 5 Notes

MQTT 5 makes a number of new features available when publishing messages, including:

• TopicAlias - Set an integer alias along with the topic name when first publishing a message. When publishing additional messages to the same topic if TopicAlias is set the Topic parameter does not need to be set. This reduces the size of the message sent to the server.
• Request / Response - Set a ResponseTopic which identifies the topic to which a receiver should respond.

Some of these features require configuration settings to be set prior to publishing. In these cases, note that these configuration settings will continue to be applicable to any future PUBLISH packets sent with the method unless they are updated or reset. To prevent clear a previously set configuration setting, set it to "", or -1 (for integers).

See the OutgoingMessageProperties configuration setting for additional details.

Reset Method (MQTT Component)

This method will reset the component.

Syntax

• C#
• VB.NET

public void Reset();

Async Version
public async Task Reset();
public async Task Reset(CancellationToken cancellationToken);

Public Sub Reset()

Async Version
Public Sub Reset() As Task
Public Sub Reset(cancellationToken As CancellationToken) As Task

Remarks

This method will reset the component's properties to their default values.

RestoreSession Method (MQTT Component)

Restores session state data.

Syntax

• C#
• VB.NET

public void RestoreSession(string stateData);

Async Version
public async Task RestoreSession(string stateData);
public async Task RestoreSession(string stateData, CancellationToken cancellationToken);

Public Sub RestoreSession(ByVal StateData As String)

Async Version
Public Sub RestoreSession(ByVal StateData As String) As Task
Public Sub RestoreSession(ByVal StateData As String, cancellationToken As CancellationToken) As Task

Remarks

This method restores previously-saved session state data and sets CleanSession to False. If the SessionStateFile configuration setting is set, the component will use the contents of that file to restore the session state data, ignoring the string passed to this method.

Depending on the elapsed time between connections the server may discard any previously saved session data on its side.

In MQTT 3.1.1, the time after which the server discards saved session data is not defined in the protocol specification and may vary between implementations. As a result, when re-connecting it may not be known if the server is capable of restoring a previous session.

To determine if a session can be restored on connection, set CleanSession to False, ClientId to the same value used in the initial connection, and call ConnectTo.

When the Connected event fires, query the SessionPresent configuration setting. If SessionPresent is True the session should be restored by calling RestoreSession. If SessionPresent is False the session should not be restored so that the connection proceeds as a clean session.

Note: Not all servers support the use of SessionPresent.

Restore Session Example (Query SessionPresent) mqtt1.CleanSession = false; mqtt1.ClientId = clientId; mqtt1.OnConnected += (s, e) => { if (mqtt1.Config("SessionPresent").ToLower() == "true") mqtt1.RestoreSession(sessionStr); }; mqtt1.ConnectTo(host,port);

If it is known that the server has not discarded the session state, for instance from familiarity with the server implementation and its behavior, then a simplified procedure can be performed. Simply set ClientId to the value used in the initial connection, call RestoreSession, and then call ConnectTo.

Restore Session Example (Server Status Known) mqtt1.ClientId = clientId; mqtt1.RestoreSession(sessionStr); mqtt1.ConnectTo(host,port);

Refer to CleanSession and SaveSession for more information.

MQTT 5 Notes

In MQTT 5, the time after which the server discards saved session data is controlled by the SessionExpInterval. The server is required to store state data for the amount of time specified by this value, at which point it is required to delete it.

This feature allows the client to use its own time-based logic to determine whether it is necessary to call RestoreSession.

SaveSession Method (MQTT Component)

Saves session state data.

Syntax

• C#
• VB.NET

public string SaveSession();

Async Version
public async Task<string> SaveSession();
public async Task<string> SaveSession(CancellationToken cancellationToken);

Public Function SaveSession() As String

Async Version
Public Function SaveSession() As Task(Of String)
Public Function SaveSession(cancellationToken As CancellationToken) As Task(Of String)

Remarks

This method saves session state data currently held by the component, returning it as a string that can later be used to restore it. If the SessionStateFile configuration setting is set, the component will also write the session state data to that file (creating and/or overwriting it, as necessary).

Refer to CleanSession and RestoreSession for more information.

Subscribe Method (MQTT Component)

Subscribes the component to one or more topic filters.

Syntax

• C#
• VB.NET

public void Subscribe(string topicFilter, int QOS);

Async Version
public async Task Subscribe(string topicFilter, int QOS);
public async Task Subscribe(string topicFilter, int QOS, CancellationToken cancellationToken);

Public Sub Subscribe(ByVal TopicFilter As String, ByVal QOS As Integer)

Async Version
Public Sub Subscribe(ByVal TopicFilter As String, ByVal QOS As Integer) As Task
Public Sub Subscribe(ByVal TopicFilter As String, ByVal QOS As Integer, cancellationToken As CancellationToken) As Task

Remarks

This method subscribes the component to one or more topic filters using one or more QoS levels.

The Subscribed event will fire once for each topic filter given once the server acknowledges the subscription request(s). Keep in mind that the server is allowed to start publishing messages before it sends the acknowledgment.

It is legal to call Subscribe again for the same topic filter at any time, and pass a different QoS value at that time if desired. The Subscribed event will fire as normal when doing this.

Subscribe Examples // Subscribed event handler. mqtt1.OnSubscribed += (s, e) => { if (e.ResponseCode <= 2) Console.WriteLine("Subscribed to " + e.TopicFilter + " at QoS " + e.QOS + "."); else Console.WriteLine("Failed to subscribe to " + e.TopicFilter + "."); }; // Basic, subscribe to some topic filters, all at the same QoS level. mqtt1.Subscribe("home,home/floor1/+/temperature,home/floor2/#", 2); // A bit more advanced, subscribe to the same topic filters, but at different QoS levels. mqtt1.Config("TopicQOSArray=1,2,2"); // The 0 is ignored here since we've specified individual QoS values explicitly. mqtt1.Subscribe("home,home/floor1/+/temperature,home/floor2/#", 0);

Topic Filters

The string passed for TopicFilter must contain one or more valid topic filter strings, separated by the delimiter string specified by the TopicDelimiter configuration setting (, by default).

A topic filter is a case-sensitive string between 1 and 65535 characters long (per topic filter), and can include any character other than the null character. Certain characters have special meanings:

• / - The topic level separator
• # - The multi-level wildcard (zero or more levels)
• + - The single-level wildcard (exactly one level)
• Leading $ - Denotes a "system topic"

Note that both types of wildcards may be used in the same topic filter.

Topic Level Separators

The topic level separator, as its name implies, is used to separate a topic name (or in this case, filter) into "levels". This concept of topic names having levels is what allows topic filters to match multiple topics through the use of wildcards. For the examples in the next sections, assume the following topics exist:

• home/floor1
• home/floor1/livingRoom
• home/floor1/livingRoom/temperature
• home/floor1/kitchen/temperature
• home/floor1/kitchen/fridge/temperature
• home/floor2/bedroom1
• home/floor2/bedroom1/temperature

Multi-level Wildcards

The multi-level wildcard character is used at the end of a topic filter to make it match an arbitrary number of successive levels. For example, the topic filter home/floor1/# would match the following topics:

• home/floor1 (because it can match zero levels)
• home/floor1/livingRoom
• home/floor1/livingRoom/temperature
• home/floor1/kitchen/temperature
• home/floor1/kitchen/fridge/temperature

Here are some things to keep in mind when using a multi-level wildcard:

• # must always be the last character in the topic filter (e.g., home/floor1/#/livingRoom is not valid)
• # must always be preceded by a / (e.g., home/floor1# is not valid)
• # by itself is a valid topic filter, and will match all topics except system topics

Single-level Wildcards

The single-level wildcard character is used between two /s in a topic filter to make it any single level. For example, the topic filter home/floor1/+/temperature would match the following topics:

• home/floor1/livingRoom/temperature
• home/floor1/kitchen/temperature

Any number of single-level wildcards are supported in a topic filter. For example, the topic filter home/+/+/temperature would match the following topics:

• home/floor1/livingRoom/temperature
• home/floor1/kitchen/temperature
• home/floor2/bedroom1/temperature

Here are some things to keep in mind when using single-level wildcards:

• + must always be separated from other levels using /s (e.g., home/floor1+ is invalid, but +/floor1/+ is valid)
• + by itself is a valid topic filter, and will match all topics with exactly one level in their name except system topics
• Remember, topic names with a leading / have a zero-length string as their first level. So a topic named /people would be matched by the topic filter +/+, but not by +
• + must match exactly one level. So for example, the topic filter home/floor1/kitchen/+/temperature would match /home/floor1/kitchen/fridge/temperature, but not home/floor1/kitchen/temperature

System Topics

Topic names which begin with a $ are "system topics". Typically, the server prohibits clients from publishing to such topics, but permits subscribing to them. As described above, wildcards will never match the first level of a topic if it begins with a $.

Note that MQTT 5 defines shared subscriptions, a special type of system topic. See the MQTT 5 notes below for more.

Requested QoS Values

The QoS value passed for QOS will be used to subscribe to all given topic filters, unless the TopicQOSArray configuration setting is used to specify explicit QoS values for each individual topic filter.

QoS values range from 0 to 2 and have the following meanings:

QoS is not guaranteed to be end-to-end in MQTT. The server must downgrade a message's QoS level when delivering it to clients who specified a lower "maximum acceptable" QoS when they subscribed. For example, if Client X subscribes to a topic at QoS 1, and Client Y publishes a message to that topic at QoS 2, the server will downgrade the message to QoS 1 when attempting to deliver it to Client X.

MQTT 5 Notes

MQTT 5 defines several new features that apply to subscriptions, including Shared Subscriptions, Subscription Options and Subscription Identifiers. Some of these features require configuration settings to be set prior to calling Subscribe. In these cases, note that thee configuration settings will continue to be applied to any future SUBSCRIBE packets sent with the method unless they are updated or reset. To prevent a value from being included in packets after it has been set previously, set it to "", or -1 (for integers).

Shared Subscriptions

Shared subscriptions allow a client to subscribe to a topic as a part of a group of clients, where each client in the group is subscribing to the same topic. Each message published to such a group will be delivered to only one client in the group, making shared subscriptions useful when several clients share the processing of the publications in parallel.

Subscribing to a topic as part of a shared subscription is similar to a normal subscription, the only difference being the syntax of the topic filter passed to the Subscribe method.

The format of this filter is $share/{ShareName}/{filter}, where

• $share is a string literal identifying the subscription as a shared subscriptions.
• {ShareName} is a string at least one character long that must not include /, + or #.
• {filter} is a full topic filter formatted just as in a non-shared subscription.

Unsubscribing is done by passing the same value to the Unsubscribe method.

A shared subscription behaves the same way as a non-shared subscription, except that the $share/{ShareName} portion of the topic filter is ignored when filtering publication topics, and that the server will distribute delivery of messages amongst the group, with each message delivered to only one client per group per topic.

Shared subscriptions can be implemented without any additional properties or methods, simply by using the Subscribe and Unsubscribe methods. However, if it is necessary to verify that the feature is supported by the server, check the "SharedSubscriptionAvailable" value of the ConnAckProperties config.

Shared Subscription Example mqtt1.Subscribe("$share/myShareGroup/myTopicFilter/subTopic", 2);

Subscription Options

MQTT 5 defines three subscription options which can be configured on a per-subscription basis and are useful for message bridge applications:

• No Local flag - indicates that messages must not be forwarded to a connection with a ClientId equal to the ClientId of the publishing connection (cannot be set on a shared subscription).
• Retain As Published flag - indicates that messages forwarded using this subscription keep the retain flag they were published with (as opposed to always being set to False).
• Retain Handling option - specifies whether retained messages are sent when the subscription is established. Possible values are:

  0	Send retained messages at the time of the subscribe
  1	Send retained messages at subscribe only if the subscription does not currently exist
  2	Do not send retained messages at the time of the subscribe

To specify these options in the subscribe method, the component has three corresponding configuration settings (TopicNLArray, TopicRAPArray and TopicRHArray) which each take a string value in the form of a comma-separated list. Each element in the list corresponds to a topic filter passed to the Subscribe method, as with TopicQOSArray.

If the subscription is only for one topic, these lists would have only one value. If the number of elements in the string is less than the number of topic filters in the subscription, the final element will be applied to all remaining topics.

Subscription Options Example mqtt1.Connected = true; mqtt1.Config("TopicNLArray=true, false, true"); // TestTopic1 = true, TestTopic2 = false, TestTopic3 = true mqtt1.Config("TopicRAPArray=false"); // TestTopic1 = false, TestTopic2 = false, TestTopic3 = false mqtt1.Config("TopicRHArray=1, 0"); // // TestTopic1 = 1, TestTopic2 = 0, TestTopic3 = 0 mqtt1.Subscribe("TestTopic1, TestTopic2, TestTopic3", 1);

Subscription Identifiers

MQTT 5 allows clients to specify a numeric subscription identifier which will be returned with messages delivered for that subscription.

To do so with the component, set the SubscriptionIdentifier configuration setting before calling Subscribe. To update the identifier, call Subscribe with a new SubscriptionIdentifier.

To verify that a server supports subscription identifiers, check the "SubscriptionIdentifiersAvailable" value in the ConnAckProperties configuration setting. See the SubscriptionIdentifiers property of the MQTTMessage type for details on returning the ids associated with messages.

Note that a topic may have multiple identifiers associated with it (due to wildcards), and the same identifier may be associated with multiple topics.

Subscription Options Example mqtt1.Connected = true; mqtt1.OnMessageAck += (o, e) => { String ids = mqtt1.IncomingMessages[e.Index].SubscriptionIdentifiers; // ids = "123,321" }; mqtt1.Config("SubscriptionIdentifier=123"); mqtt1.Subscribe("test_topic/subtopic", 1); mqtt1.Config("SubscriptionIdentifier=321"); mqtt1.Subscribe("test_topic/+", 1); mqtt2.Connected = true; mqtt2.PublishMessage("test_topic/subtopic", 1, "hello world");

Unsubscribe Method (MQTT Component)

Unsubscribes the component from one or more topic filters.

Syntax

• C#
• VB.NET

public void Unsubscribe(string topicFilter);

Async Version
public async Task Unsubscribe(string topicFilter);
public async Task Unsubscribe(string topicFilter, CancellationToken cancellationToken);

Public Sub Unsubscribe(ByVal TopicFilter As String)

Async Version
Public Sub Unsubscribe(ByVal TopicFilter As String) As Task
Public Sub Unsubscribe(ByVal TopicFilter As String, cancellationToken As CancellationToken) As Task

Remarks

This method unsubscribes the component from one or more topic filters. The string passed for TopicFilter must contain one or more valid topic filter strings, separated by the delimiter string specified by the TopicDelimiter configuration setting (, by default).

The Unsubscribed event will fire when the server has acknowledged the unsubscribe request. However, note that the server's acknowledgment doesn't specify which topics a client was unsubscribed from. Please refer to the Unsubscribed event for more information.

To successfully unsubscribe from a topic filter, the exact filter must be passed to Unsubscribe, regardless of whether or not if contains wildcards. The server ignores any topic filters in an unsubscribe request which do not exactly match that of an existing subscription.

It is impossible to partially unsubscribe from a topic filter with wildcards (that is, if a client is subscribed to a topic filter home/floor1/+/#, requesting to unsubscribe from a topic filter home/floor1/livingRoom/temperature does nothing).

Similarly, because topic filters in an unsubscribe request are simply compared character-by-character with existing subscriptions rather than being interpreted, it is not possible to perform an action such as unsubscribing from all currently subscribed topics by passing "#" for TopicFilter.

Unsubscribe Example // Unsubscribe from topic filters; have to use the exact same strings as before. If this // was to be called after calling the code example shown for the Subscribe() method, we // would still be subscribed to the "home" topic filter. mqtt1.Unsubscribe("home/floor1/+/temperature,home/floor2/#");

Connected Event (MQTT Component)

Fired immediately after a connection completes (or fails).

Syntax

• C#
• VB.NET

public event OnConnectedHandler OnConnected;

public delegate void OnConnectedHandler(object sender, MQTTConnectedEventArgs e);

public class MQTTConnectedEventArgs : EventArgs {
  public int StatusCode { get; }
  public string Description { get; }
}

Public Event OnConnected As OnConnectedHandler

Public Delegate Sub OnConnectedHandler(sender As Object, e As MQTTConnectedEventArgs)

Public Class MQTTConnectedEventArgs Inherits EventArgs
  Public ReadOnly Property StatusCode As Integer
  Public ReadOnly Property Description As String
End Class

Remarks

If the connection is made normally, StatusCode is 0 and Description is "OK".

If the connection fails, StatusCode has the error code returned by the Transmission Control Protocol (TCP)/IP stack. Description contains a description of this code. The value of StatusCode is equal to the value of the error.

Please refer to the Error Codes section for more information.

ConnectionStatus Event (MQTT Component)

Fired to indicate changes in the connection state.

Syntax

• C#
• VB.NET

public event OnConnectionStatusHandler OnConnectionStatus;

public delegate void OnConnectionStatusHandler(object sender, MQTTConnectionStatusEventArgs e);

public class MQTTConnectionStatusEventArgs : EventArgs {
  public string ConnectionEvent { get; }
  public int StatusCode { get; }
  public string Description { get; }
}

Public Event OnConnectionStatus As OnConnectionStatusHandler

Public Delegate Sub OnConnectionStatusHandler(sender As Object, e As MQTTConnectionStatusEventArgs)

Public Class MQTTConnectionStatusEventArgs Inherits EventArgs
  Public ReadOnly Property ConnectionEvent As String
  Public ReadOnly Property StatusCode As Integer
  Public ReadOnly Property Description As String
End Class

Remarks

This event is fired when the connection state changes: for example, completion of a firewall or proxy connection or completion of a security handshake.

The ConnectionEvent parameter indicates the type of connection event. Values may include the following:

Disconnected Event (MQTT Component)

Fired when a connection is closed.

Syntax

• C#
• VB.NET

public event OnDisconnectedHandler OnDisconnected;

public delegate void OnDisconnectedHandler(object sender, MQTTDisconnectedEventArgs e);

public class MQTTDisconnectedEventArgs : EventArgs {
  public int StatusCode { get; }
  public string Description { get; }
}

Public Event OnDisconnected As OnDisconnectedHandler

Public Delegate Sub OnDisconnectedHandler(sender As Object, e As MQTTDisconnectedEventArgs)

Public Class MQTTDisconnectedEventArgs Inherits EventArgs
  Public ReadOnly Property StatusCode As Integer
  Public ReadOnly Property Description As String
End Class

Remarks

If the connection is broken normally, StatusCode is 0 and Description is "OK".

If the connection is broken for any other reason, StatusCode has the error code returned by the Transmission Control Protocol (TCP/IP) subsystem. Description contains a description of this code. The value of StatusCode is equal to the value of the TCP/IP error.

Please refer to the Error Codes section for more information.

Error Event (MQTT Component)

Fired when information is available about errors during data delivery.

Syntax

• C#
• VB.NET

public event OnErrorHandler OnError;

public delegate void OnErrorHandler(object sender, MQTTErrorEventArgs e);

public class MQTTErrorEventArgs : EventArgs {
  public int ErrorCode { get; }
  public string Description { get; }
}

Public Event OnError As OnErrorHandler

Public Delegate Sub OnErrorHandler(sender As Object, e As MQTTErrorEventArgs)

Public Class MQTTErrorEventArgs Inherits EventArgs
  Public ReadOnly Property ErrorCode As Integer
  Public ReadOnly Property Description As String
End Class

Remarks

The Error event is fired in case of exceptional conditions during message processing. Normally the component throws an exception.

The ErrorCode parameter contains an error code, and the Description parameter contains a textual description of the error. For a list of valid error codes and their descriptions, please refer to the Error Codes section.

Log Event (MQTT Component)

Fires once for each log message.

Syntax

• C#
• VB.NET

public event OnLogHandler OnLog;

public delegate void OnLogHandler(object sender, MQTTLogEventArgs e);

public class MQTTLogEventArgs : EventArgs {
  public int LogLevel { get; }
  public string Message { get; }
  public string LogType { get; }
}

Public Event OnLog As OnLogHandler

Public Delegate Sub OnLogHandler(sender As Object, e As MQTTLogEventArgs)

Public Class MQTTLogEventArgs Inherits EventArgs
  Public ReadOnly Property LogLevel As Integer
  Public ReadOnly Property Message As String
  Public ReadOnly Property LogType As String
End Class

Remarks

This event fires once for each log message generated by the component. The verbosity is controlled by the LogLevel setting.

LogLevel indicates the level of the Message. Possible values are:

LogType identifies the type of log entry. Possible values are:

• Info: General information about the component.
• Packet: Packet content logging.
• Reconnect: Reconnection status messages.
• Session: Session status messages.

MessageAck Event (MQTT Component)

Fired when an incoming or outgoing message has completed all acknowledgment steps.

Syntax

• C#
• VB.NET

public event OnMessageAckHandler OnMessageAck;

public delegate void OnMessageAckHandler(object sender, MQTTMessageAckEventArgs e);

public class MQTTMessageAckEventArgs : EventArgs {
  public int PacketId { get; }
  public int Direction { get; }
  public int Index { get; }
  public int ResponseCode { get; }
}

Public Event OnMessageAck As OnMessageAckHandler

Public Delegate Sub OnMessageAckHandler(sender As Object, e As MQTTMessageAckEventArgs)

Public Class MQTTMessageAckEventArgs Inherits EventArgs
  Public ReadOnly Property PacketId As Integer
  Public ReadOnly Property Direction As Integer
  Public ReadOnly Property Index As Integer
  Public ReadOnly Property ResponseCode As Integer
End Class

Remarks

The MessageAck event fires once an incoming or outgoing message with a QoS of 1 or 2 has successfully completed all acknowledgment steps as required by its QoS level. (Note that this event does not fire for messages with a QoS of 0 since they do not require acknowledgment.)

• PacketId: The Id of the original PUBLISH packet for the message.
• Direction: Shows whether the client (0) or the server (1) is sending the data.
• Index: The index at which the message resides in either the IncomingMessages collection or OutgoingMessages collection.
• ResponseCode: In MQTT 5, all response packets contain Reason Codes. This argument contains any code encountered in message acknowledgment packets (PUBACK, PUBREC, PUBREL, PUBCOMP).

Possible MQTT-specific values for ResponseCode are:

Inbound Message Processing

Incoming messages with a QoS of 1 follow these steps:

1. The message is added to IncomingMessages when the component receives the PUBLISH packet.
2. The component sends a PUBACK (publish acknowledgment) packet in response.
3. The MessageAck event is fired.
4. The message is removed from IncomingMessages.
5. The MessageIn event is fired.

Incoming messages with a QoS of 2 follow these steps:

1. The message is added to IncomingMessages when the component receives the PUBLISH packet.
2. The component sends a PUBREC (publish received) packet in response.
3. The component waits to receive a PUBREL (publish release) packet.
4. The component sends a PUBCOMP (publish complete) packet in response.
5. The MessageAck event is fired.
6. The message is removed from IncomingMessages.
7. The MessageIn event is fired.

Outbound Message Processing

Outgoing messages with a QoS of 1 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBACK (publish acknowledgment) packet.
3. The MessageAck event is fired.
4. The message is removed from OutgoingMessages.
5. The MessageOut event is fired.

Outgoing messages with a QoS of 2 follow these steps:

1. The component sends the PUBLISH packet, then adds the message to OutgoingMessages.
2. The component waits to receive a PUBREC (publish received) packet.
3. The component sends a PUBREL (publish release) packet in response.
4. The component waits to receive a PUBCOMP (publish complete) packet.
5. The MessageAck event is fired.
6. The message is removed from OutgoingMessages.
7. The MessageOut event is fired.

In MQTT 3.1.1, the RepublishInterval configuration setting, if set to a non-zero value (default), controls how long the component will wait to receive a PUBACK (for QoS 1) or PUBREC (for QoS 2) before automatically republishing an outgoing message. In MQTT 5, messages are only republished if the client is disconnected before receiving a PUBACK or PUBREC.

MessageIn Event (MQTT Component)

Fired when an incoming message has been received and/or fully acknowledged.

Syntax

• C#
• VB.NET

public event OnMessageInHandler OnMessageIn;

public delegate void OnMessageInHandler(object sender, MQTTMessageInEventArgs e);

public class MQTTMessageInEventArgs : EventArgs {
  public int PacketId { get; }
  public string Topic { get; }
  public int QOS { get; }
  public string Message { get; }
  public byte[] MessageB { get; }
  public bool Retained { get; }
  public bool Duplicate { get; }
}

Public Event OnMessageIn As OnMessageInHandler

Public Delegate Sub OnMessageInHandler(sender As Object, e As MQTTMessageInEventArgs)

Public Class MQTTMessageInEventArgs Inherits EventArgs
  Public ReadOnly Property PacketId As Integer
  Public ReadOnly Property Topic As String
  Public ReadOnly Property QOS As Integer
  Public ReadOnly Property Message As String
  Public ReadOnly Property MessageB As Byte()
  Public ReadOnly Property Retained As Boolean
  Public ReadOnly Property Duplicate As Boolean
End Class

Remarks

The MessageIn event fires once for each incoming message either immediately after it is received (QoS 0), or after it has been fully acknowledged (QoS 1 and 2).

• PacketId: The message packet Id. This will always be -1 if QOS is 0.
• Topic: The message's topic string.
• QOS: The message's QoS level.
• Message: The message data.
• Retained: Whether or not this message was received as a result of subscribing to a topic.
• Duplicate: Whether or not the server has indicated that this message is a duplicate of another message sent previously.

Refer to MessageAck for more information about QoS 1 and 2 message processing steps.

// MessageIn event handler. mqtt1.OnMessageIn += (s, e) => { Console.WriteLine("Received message from topic '" + e.Topic + "' with QoS " + e.QOS + ":"); Console.WriteLine(e.Message); };

MessageOut Event (MQTT Component)

Fired when an outgoing message has been sent and/or fully acknowledged.

Syntax

• C#
• VB.NET

public event OnMessageOutHandler OnMessageOut;

public delegate void OnMessageOutHandler(object sender, MQTTMessageOutEventArgs e);

public class MQTTMessageOutEventArgs : EventArgs {
  public int PacketId { get; }
  public string Topic { get; }
  public int QOS { get; }
  public string Message { get; }
  public byte[] MessageB { get; }
  public bool Retained { get; }
  public bool Duplicate { get; }
}

Public Event OnMessageOut As OnMessageOutHandler

Public Delegate Sub OnMessageOutHandler(sender As Object, e As MQTTMessageOutEventArgs)

Public Class MQTTMessageOutEventArgs Inherits EventArgs
  Public ReadOnly Property PacketId As Integer
  Public ReadOnly Property Topic As String
  Public ReadOnly Property QOS As Integer
  Public ReadOnly Property Message As String
  Public ReadOnly Property MessageB As Byte()
  Public ReadOnly Property Retained As Boolean
  Public ReadOnly Property Duplicate As Boolean
End Class

Remarks

The MessageOut event fires once for each outgoing message either immediately after it is sent (QoS 0), or after it has been fully acknowledged by the receiver (QoS 1 and 2).

• PacketId: The message packet Id. This will always be -1 if QOS is 0.
• Topic: The message's topic string.
• QOS: The message's QoS level.
• Message: The message data.
• Retained: Whether or not this message was sent with the "Retain" flag set.
• Duplicate: Whether or not this message was sent with the "Duplicate" flag set.

Refer to MessageAck for more information about QoS 1 and 2 message processing steps.

// MessageOut event handler. mqtt1.OnMessageOut += (s, e) => { Console.WriteLine("Send message to topic '" + e.Topic + "' with QoS " + e.QOS + ":"); Console.WriteLine(e.Message); };

ReadyToSend Event (MQTT Component)

Fired when the component is ready to send data.

Syntax

• C#
• VB.NET

public event OnReadyToSendHandler OnReadyToSend;

public delegate void OnReadyToSendHandler(object sender, MQTTReadyToSendEventArgs e);

public class MQTTReadyToSendEventArgs : EventArgs {
}

Public Event OnReadyToSend As OnReadyToSendHandler

Public Delegate Sub OnReadyToSendHandler(sender As Object, e As MQTTReadyToSendEventArgs)

Public Class MQTTReadyToSendEventArgs Inherits EventArgs
End Class

Remarks

The ReadyToSend event indicates that the underlying TCP/IP subsystem is ready to accept data after a call to PublishData or PublishMessage fails due to a WOULDBLOCK condition. The event is also fired immediately after a connection to the remote host is established.

SSLServerAuthentication Event (MQTT Component)

Fired after the server presents its certificate to the client.

Syntax

• C#
• VB.NET

public event OnSSLServerAuthenticationHandler OnSSLServerAuthentication;

public delegate void OnSSLServerAuthenticationHandler(object sender, MQTTSSLServerAuthenticationEventArgs e);

public class MQTTSSLServerAuthenticationEventArgs : EventArgs {
  public string CertEncoded { get; }
  public byte[] CertEncodedB { get; }
  public string CertSubject { get; }
  public string CertIssuer { get; }
  public string Status { get; }
  public bool Accept { get; set; }
}

Public Event OnSSLServerAuthentication As OnSSLServerAuthenticationHandler

Public Delegate Sub OnSSLServerAuthenticationHandler(sender As Object, e As MQTTSSLServerAuthenticationEventArgs)

Public Class MQTTSSLServerAuthenticationEventArgs Inherits EventArgs
  Public ReadOnly Property CertEncoded As String
  Public ReadOnly Property CertEncodedB As Byte()
  Public ReadOnly Property CertSubject As String
  Public ReadOnly Property CertIssuer As String
  Public ReadOnly Property Status As String
  Public Property Accept As Boolean
End Class

Remarks

During this event, the client can decide whether or not to continue with the connection process. The Accept parameter is a recommendation on whether to continue or close the connection. This is just a suggestion: application software must use its own logic to determine whether or not to continue.

When Accept is False, Status shows why the verification failed (otherwise, Status contains the string OK). If it is decided to continue, you can override and accept the certificate by setting the Accept parameter to True.

SSLStatus Event (MQTT Component)

Fired when secure connection progress messages are available.

Syntax

• C#
• VB.NET

public event OnSSLStatusHandler OnSSLStatus;

public delegate void OnSSLStatusHandler(object sender, MQTTSSLStatusEventArgs e);

public class MQTTSSLStatusEventArgs : EventArgs {
  public string Message { get; }
}

Public Event OnSSLStatus As OnSSLStatusHandler

Public Delegate Sub OnSSLStatusHandler(sender As Object, e As MQTTSSLStatusEventArgs)

Public Class MQTTSSLStatusEventArgs Inherits EventArgs
  Public ReadOnly Property Message As String
End Class

Remarks

The event is fired for informational and logging purposes only. This event tracks the progress of the connection.

Subscribed Event (MQTT Component)

Fires for each topic filter subscription the server acknowledges.

Syntax

• C#
• VB.NET

public event OnSubscribedHandler OnSubscribed;

public delegate void OnSubscribedHandler(object sender, MQTTSubscribedEventArgs e);

public class MQTTSubscribedEventArgs : EventArgs {
  public string TopicFilter { get; }
  public int QOS { get; }
  public int ResponseCode { get; }
}

Public Event OnSubscribed As OnSubscribedHandler

Public Delegate Sub OnSubscribedHandler(sender As Object, e As MQTTSubscribedEventArgs)

Public Class MQTTSubscribedEventArgs Inherits EventArgs
  Public ReadOnly Property TopicFilter As String
  Public ReadOnly Property QOS As Integer
  Public ReadOnly Property ResponseCode As Integer
End Class

Remarks

This event fires each time the server has acknowledged a topic filter subscription request (that is, calling Subscribe with multiple topic filters will cause this to fire multiple times).

• TopicFilter: The topic filter subscription request being acknowledged.
• QOS: The QoS level the server has granted for the subscription.
• ResponseCode: Indicates the result of the subscription request.

Possible values for ResponseCode are:

• 0: Success, QoS 0 granted
• 1: Success, QoS 1 granted
• 2: Success, QoS 2 granted
• 128: Unspecified error
• 131: Implementation specific error; The SUBSCRIBE is valid but the Server does not accept it.
• 135: Not authorized; The Client is not authorized to make this subscription.
• 143: Topic Filter invalid; The Topic Filter is correctly formed but is not allowed for this Client.
• 145: Packet Identifier in use
• 151: Quota exceeded; An implementation or administrative imposed limit has been exceeded.
• 158: Shared Subscriptions not supported
• 161: Shared Subscriptions not supported
• 162: Wildcard Subscriptions not supported

Keep in mind that the server may have chosen to grant a lower QoS than was requested under certain circumstances (e.g., if the server doesn't support the requested QoS).

Note: the server is not required to acknowledge a subscription before it begins delivering messages for that subscription.

Unsubscribed Event (MQTT Component)

Fires when the server has acknowledged an unsubscribe request.

Syntax

• C#
• VB.NET

public event OnUnsubscribedHandler OnUnsubscribed;

public delegate void OnUnsubscribedHandler(object sender, MQTTUnsubscribedEventArgs e);

public class MQTTUnsubscribedEventArgs : EventArgs {
  public string TopicFilters { get; }
  public int ResponseCode { get; }
}

Public Event OnUnsubscribed As OnUnsubscribedHandler

Public Delegate Sub OnUnsubscribedHandler(sender As Object, e As MQTTUnsubscribedEventArgs)

Public Class MQTTUnsubscribedEventArgs Inherits EventArgs
  Public ReadOnly Property TopicFilters As String
  Public ReadOnly Property ResponseCode As Integer
End Class

Remarks

This event fires when the server has acknowledged an unsubscribe request sent with Unsubscribe (that is, unlike the Subscribed event, this event fires only once for each call to Unsubscribe, regardless of how many topic filters are passed to it).

Note that in MQTT 3.1.1, servers do not specify which topic filters a client has been successfully unsubscribed in their acknowledgments. In fact, they will still acknowledge an unsubscribe request even if none of the topic filters included matched existing subscriptions. It is up to the client to keep track of what topics it is subscribed to; the TopicFilters parameter is provided as a convenience, its value simply a copy of the value passed to Unsubscribe originally.

Possible values for ResponseCode are:

• 0: Success; The subscription is deleted.
• 17: No subscription existed; No matching Topic Filter is being used by the Client.
• 128: Unspecified error; The unsubscribe could not be completed and the Server either does not wish to reveal the reason or none of the other Reason Codes apply.
• 131: Implementation specific error; The UNSUBSCRIBE is valid but the Server does not accept it.
• 135: Not authorized; The Client is not authorized to unsubscribe.
• 143: Topic Filter invalid; The Topic Filter is correctly formed but is not allowed for this Client.
• 145: Packet Identifier in use; The specified Packet Identifier is already in use.

Certificate Type

This is the digital certificate being used.

Remarks

This type describes the current digital certificate. The certificate may be a public or private key. The fields are used to identify or select certificates.

Fields

Constructors

• C#
• VB.NET

public Certificate();

Public Certificate()

• C#
• VB.NET

public Certificate(string certificateFile);

Public Certificate(ByVal CertificateFile As String)

• C#
• VB.NET

public Certificate(byte[] encoded);

Public Certificate(ByVal Encoded As Byte())

• C#
• VB.NET

public Certificate(CertStoreTypes storeType, string store, string storePassword, string subject);

Public Certificate(ByVal StoreType As CertStoreTypes, ByVal Store As String, ByVal StorePassword As String, ByVal Subject As String)

• C#
• VB.NET

public Certificate(CertStoreTypes storeType, string store, string storePassword, string subject, string configurationString);

Public Certificate(ByVal StoreType As CertStoreTypes, ByVal Store As String, ByVal StorePassword As String, ByVal Subject As String, ByVal ConfigurationString As String)

• C#
• VB.NET

public Certificate(CertStoreTypes storeType, string store, string storePassword, byte[] encoded);

Public Certificate(ByVal StoreType As CertStoreTypes, ByVal Store As String, ByVal StorePassword As String, ByVal Encoded As Byte())

• C#
• VB.NET

public Certificate(CertStoreTypes storeType, byte[] store, string storePassword, string subject);

Public Certificate(ByVal StoreType As CertStoreTypes, ByVal Store As Byte(), ByVal StorePassword As String, ByVal Subject As String)

• C#
• VB.NET

public Certificate(CertStoreTypes storeType, byte[] store, string storePassword, string subject, string configurationString);

Public Certificate(ByVal StoreType As CertStoreTypes, ByVal Store As Byte(), ByVal StorePassword As String, ByVal Subject As String, ByVal ConfigurationString As String)

• C#
• VB.NET

public Certificate(CertStoreTypes storeType, byte[] store, string storePassword, byte[] encoded);

Public Certificate(ByVal StoreType As CertStoreTypes, ByVal Store As Byte(), ByVal StorePassword As String, ByVal Encoded As Byte())

Firewall Type

The firewall the component will connect through.

Remarks

When connecting through a firewall, this type is used to specify different properties of the firewall, such as the firewall Host and the FirewallType.

Fields

Constructors

• C#
• VB.NET

public Firewall();

Public Firewall()

MQTTMessage Type

An MQTT message.

Remarks

This type represents an MQTT message.

Fields

Config Settings (MQTT Component)

MQTT Config Settings

{ 
  "ClientTopicAliasMax":"20",
  "MaximumPacketSize":"128000",
  "ReceiveMaximum":"12",
  "RequestResponseInformation":"false",
  "RequestProblemInformation":"true",
  "SessionExpInterval":"1234"
}

{
  "ContentType":"plain/text",
  "CorrelationData":"00000000",
  "MessageExpInterval":"1000",
  "PayloadFormatIndicator":"1",
  "ResponseTopic": "ResponseTopic",
  "TopicAlias":"1",
  "UserProperty":"prp1:val1",
  "UserProperty":"prp2:val2"
}

{
"ContentType":"text/plain",
"CorrelationData":"00000000",
"MessageExpInterval":"3600",
"PayloadFormatIndicator":"1",
"ResponseTopic": "ResponseTopic",
"WillDelayInterval":"10"
}

TCPClient Config Settings

SSL Config Settings

-----BEGIN CERTIFICATE-----
MIIEKzCCAxOgAwIBAgIRANTET4LIkxdH6P+CFIiHvTowDQYJKoZIhvcNAQELBQAw
... Intermediate Cert ...
eWHV5OW1K53o/atv59sOiW5K3crjFhsBOd5Q+cJJnU+SWinPKtANXMht+EDvYY2w
F0I1XhM+pKj7FjDr+XNj
-----END CERTIFICATE-----
\r \n
-----BEGIN CERTIFICATE-----
MIIEFjCCAv6gAwIBAgIQetu1SMxpnENAnnOz1P+PtTANBgkqhkiG9w0BAQUFADBp
... Root Cert ...
d8q23djXZbVYiIfE9ebr4g3152BlVCHZ2GyPdjhIuLeH21VbT/dyEHHA
-----END CERTIFICATE-----

-----BEGIN CERTIFICATE-----
MIIEKzCCAxOgAwIBAgIRANTET4LIkxdH6P+CFIiHvTowDQYJKoZIhvcNAQELBQAw
... Intermediate Cert ...
eWHV5OW1K53o/atv59sOiW5K3crjFhsBOd5Q+cJJnU+SWinPKtANXMht+EDvYY2w
F0I1XhM+pKj7FjDr+XNj
-----END CERTIFICATE-----
\r \n
-----BEGIN CERTIFICATE-----
MIIEFjCCAv6gAwIBAgIQetu1SMxpnENAnnOz1P+PtTANBgkqhkiG9w0BAQUFADBp
... Root Cert ...
d8q23djXZbVYiIfE9ebr4g3152BlVCHZ2GyPdjhIuLeH21VbT/dyEHHA
-----END CERTIFICATE-----

-----BEGIN CERTIFICATE-----
MIIEKzCCAxOgAwIBAgIRANTET4LIkxdH6P+CFIiHvTowDQYJKoZIhvcNAQELBQAw
... Intermediate Cert...
eWHV5OW1K53o/atv59sOiW5K3crjFhsBOd5Q+cJJnU+SWinPKtANXMht+EDvYY2w
F0I1XhM+pKj7FjDr+XNj
-----END CERTIFICATE-----
\r \n
-----BEGIN CERTIFICATE-----
MIIEFjCCAv6gAwIBAgIQetu1SMxpnENAnnOz1P+PtTANBgkqhkiG9w0BAQUFADBp
... Root Cert...
d8q23djXZbVYiIfE9ebr4g3152BlVCHZ2GyPdjhIuLeH21VbT/dyEHHA
-----END CERTIFICATE-----

Socket Config Settings

Trappable Errors (MQTT Component)

MQTT Errors

WebSocket Errors

HTTP Errors

The component may also return one of the following error codes, which are inherited from other components.

TCPClient Errors

SSL Errors

TCP/IP Errors