Saves session state data.
This method saves session state data currently held by the class, returning it as a string that can later be used to restore it. If the SessionStateFile configuration setting is set, the class will also write the session state data to that file (creating and/or overwriting it, as necessary).
This method returns a String value; after it returns, call the GetLastErrorCode() method to obtain its result code; 0 indicates success, while a non-zero error code indicates that this method encountered an error during its execution. If an error occurs, the GetLastError() method can be called to retrieve the associated error message.