Receive WM_COPYDATA messages in a Qt app - c++

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:
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 );
};
Generate a GUID to set as the QWidget's windowTitle:
EventReceiverWindow::EventReceiverWindow()
{
setWindowTitle("ProjectName-3F2504E0-4F89-11D3-9A0C-0305E82C3301::EventReceiver");
}
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;
}
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;
}
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");

Answer 2

You can also create a dummy window just for receiving that message with the Win32 API. I guess you won't have access to a Qt-Window's window proc, so this should be the easiest way.
You could (I wouldn't) also subclass the window by setting a new WndProc (with SetWindowLong(Ptr), the window's handle can be obtained with QWidget::winId()). In this WndProc, you could just handle your specific WM_COPYDATA and pass all other window messages to the old WndProc.

Answer 3

To handle messages your window receives, override your QCoreApplication::winEventFilter. If that doesn't work you can take a look at QAbstractEventDispatcher.
For the class name you could try using QWidget::winId along with Win32 API. I would try and find it for you but I can't right now, maybe try GetClassName.

Answer 4

You can use QWinHost from Qt solutions to create a dummy window. Following the guide will show you how to specify your class name and check the event loop for your message.