Receive WM_COPYDATA messages in a Qt app

Question

I am working on a Windows-only Qt application, and I need to receive data from a Microsoft OneNote plugin. The plugin is written in C#, and can send WM_COPYDATA messages. How do I receive these messages in a C++ Qt app?

I need to:

• Be able to specify the "class name" a window registers as when it calls RegisterClassEx, so that I can make sure the plugin sends WM_COPYDATA messages to the correct window.
• Have access to the message id to check if it's WM_COPYDATA and lParam, which contains the COPYDATASTRUCT with the actual data. This information is passed in WndProc, but I am unable to find a hook where I can intercept these messages.

Answer 1

This can all be handled within Qt:

1. Extend QWidget with a class that will capture the WM_COPYDATA messages:

       class EventReceiverWindow : public QWidget
   {
       Q_OBJECT
   public:
       EventReceiverWindow();
   
   signals:
       void eventData(const QString & data);
   
   private:
       bool winEvent ( MSG * message, long * result );
   };
   

2. Generate a GUID to set as the QWidget's windowTitle:

   EventReceiverWindow::EventReceiverWindow()
   {
       setWindowTitle("ProjectName-3F2504E0-4F89-11D3-9A0C-0305E82C3301::EventReceiver");
   }
   

3. Override winEvent to handle the WM_COPYDATA structure and emit a signal when you get it:

   bool EventReceiverWindow::winEvent ( MSG * message, long * result )
   {
           if( message->message == WM_COPYDATA ) {
               // extract the string from lParam
               COPYDATASTRUCT * data = (COPYDATASTRUCT *) message->lParam;
   
               emit eventData(QString::fromAscii((const char *)data->lpData, data->cbData));
   
               // keep the event from qt
               *result = 0;
               return true;
           }
   
           // give the event to qt
           return false;
   }
   

4. In another class, you can use this class to receive the message strings:

   EventReceiverWindow * eventWindow = new EventReceiverWindow;
   QObject::connect(eventWindow, SIGNAL(eventData(const QString &)), this, SLOT(handleEventData(const QString &)));
   

   ...

   void OneNoteInterface::handleEventData(const QString &data)
   {
       qDebug() << "message from our secret agent: " << data;
   }
   

5. And in the program that is sending the messages, simply find the window by the unique window caption. Here's an example in C#:

   private struct COPYDATASTRUCT
   {
       public IntPtr dwData;
       public int cbData;
       [MarshalAs(UnmanagedType.LPStr)]
       public string lpData;
   }
   
   private const int WM_COPYDATA = 0x4A;
   
   [DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
   static extern IntPtr FindWindowByCaption(IntPtr ZeroOnly, string lpWindowName);
   
   [DllImport("User32.dll", EntryPoint = "SendMessage")]
   private static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, ref COPYDATASTRUCT lParam);
   
   private void sendMessageTo(IntPtr hWnd, String msg)
   {
       int wParam = 0;
       int result = 0;
   
       if (hWnd != IntPtr.Zero )
       {
           byte[] sarr = System.Text.Encoding.Default.GetBytes(msg);
           int len = sarr.Length;
           COPYDATASTRUCT cds;
           cds.dwData = IntPtr.Zero;
           cds.lpData = msg;
           cds.cbData = len + 1;
           result = SendMessage(hWnd, WM_COPYDATA, wParam, ref cds);
       }
   }
   

   then you can:

   IntPtr hwnd = FindWindowByCaption(IntPtr.zero, "ProjectName-3F2504E0-4F89-11D3-9A0C-0305E82C3301::EventReceiver");
   sendMessageTo(hwnd, "omg hai");