struct OpalMessage {
  OpalMessageType m_type;   ///< Type of message
  union {
    const char *             m_commandError;       ///< Used by OpalIndCommandError
    OpalParamGeneral         m_general;            ///< Used by OpalCmdSetGeneralParameters
    OpalParamProtocol        m_protocol;           ///< Used by OpalCmdSetProtocolParameters
    OpalParamRegistration    m_registrationInfo;   ///< Used by OpalCmdRegistration
    OpalStatusRegistration   m_registrationStatus; ///< Used by OpalIndRegistration
    OpalParamSetUpCall       m_callSetUp;          ///< Used by OpalCmdSetUpCall/OpalIndProceeding/OpalIndAlerting/OpalIndEstablished
...
  } m_param;
};
===
  OpalMessage * OPAL_EXPORT OpalSendMessage(OpalHandle handle, const OpalMessage * message)
  {
    return handle == NULL ? NULL : handle->manager.SendMessage(message);
  }

    [StructLayout(LayoutKind.Explicit)]
    public struct OpalMessageUnion
    {
      //string
      [FieldOffset(0)]
      public IntPtr m_commandError;       ///< Used by OpalIndCommandError

      //string
      [FieldOffset(0)]
      public IntPtr m_callToken;          ///< Used by OpalCmdHoldcall/OpalCmdRetrieveCall/OpalCmdStopRecording
                                          ///
      [FieldOffset(0)]      
      public OpalParamGeneral m_general;            ///< Used by OpalCmdSetGeneralParameters

      [FieldOffset(0)]
      public OpalParamProtocol m_protocol;           ///< Used by OpalCmdSetProtocolParameters

      [FieldOffset(0)]
      public OpalParamRegistration m_registrationInfo;   ///< Used by OpalCmdRegistration

      [FieldOffset(0)]
      public OpalStatusRegistration m_registrationStatus; ///< Used by OpalIndRegistration

      [FieldOffset(0)]
      public OpalParamSetUpCall m_callSetUp;          ///< Used by OpalCmdSetUpCall/OpalIndProceeding/OpalIndAlerting/OpalIndEstablished     
...
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct OpalMessage
    {
      /// type of message
      [FieldOffset(0)]
      public OpalMessageType m_type;

      [FieldOffset(4)]
      public OpalMessageUnion m_param;
    }
    [DllImport("OPAL.DLL", CallingConvention = CallingConvention.StdCall)]    
    public static extern IntPtr OpalSendMessage(IntPtr opal, ref OpalMessage message);

Public Type OpalMessageCommandError
  m_type As OpalMessageType 'type of message=OpalIndCommandError
  m_param As String
End Type

Public Type OpalMessageGeneral
  m_type As OpalMessageType 'type of message=OpalCmdSetGeneralParameters
  m_param As OpalParamGeneral
End Type

Public Type OpalMessageProtocol
  m_type As OpalMessageType 'type of message=OpalCmdSetProtocolParameters
  m_param As OpalParamProtocol
End Type

Public Type OpalMessageRegistration
  m_type As OpalMessageType 'type of message=OpalCmdRegistration
  m_param As OpalParamRegistration
End Type

Public Type OpalMessageStatusRegistration
  m_type As OpalMessageType 'type of message=OpalIndRegistration
  m_param As OpalStatusRegistration
End Type

Public Type OpalMessageSetUpCall
  m_type As OpalMessageType 'type of message=OpalCmdSetUpCall/OpalIndProceeding/OpalIndAlerting/OpalIndEstablished
  m_param As OpalParamSetUpCall
End Type
'===
Public Declare Function OpalSendMessage_SetGeneralParameters Lib "opal.dll" Alias "OpalSendMessage" _
  (ByVal opal As Long, ByRef Message As OpalMessageGeneral) As Long
Public Declare Function OpalSendMessage_SetProtocolParameters Lib "opal.dll" Alias "OpalSendMessage" _
  (ByVal opal As Long, ByRef Message As OpalMessageProtocol) As Long
Public Declare Function OpalSendMessage_Registration Lib "opal.dll" Alias "OpalSendMessage" _
  (ByVal opal As Long, ByRef Message As OpalMessageRegistration) As Long
Public Declare Function OpalSendMessage_SetUpCall Lib "opal.dll" Alias "OpalSendMessage" _
  (ByVal opal As Long, ByRef Message As OpalMessageSetUpCall) As Long