asked on

Sending a Windows Message in DLL

Hi,

I have the following code to send a custom message to an application. This code is in a exported DLL function that is called by the system.
But the message never gets sent. What am I doing wrong?

WINLOGONDLL_API UINT SendTestMessage()
{

      UINT iMsg = RegisterWindowMessage(MY_MESSAGE_UNLOCK);
      SendMessage(hWnd, iMsg, NULL, NULL);

      return iMsg;
}

MSDN says following gor SendMessage()
"The system only does marshalling for system messages (those in the range 0 to WM_USER). To send other messages (those above WM_USER) to another process, you must do custom marshalling."

Do I need to do custom marshaling? How do i do that?

Thanks
Alex

jkr

>>Do I need to do custom marshaling?

As you are not passing anything in the parameters: No. Are you sure the window handle is valid? Have you checked the return value of 'RegisterWindowMessage()'?

astankovic

ASKER

Hi, thanks for your comment. Yes, I checked the RegisterWindowMessage(), I get a value of 49946

I'm now trying to verify if I have a valid handle. I have another exported DLL function:

// hWnd is global variable that gets passed to SendMessage()
WINLOGONDLL_API HWND SetWindow(HWND appWindow)
{
hWnd = appWindow;

return hWnd;
}

This function is being called by a C# app when it starts. As you suggested, I think there is something wrong going in that part.
In C# app i do following:
MessageBox.Show(this.Handle.ToString()); //window handle
MessageBox.Show(SetWindow(this.Handle).ToString()); //window handle returned by the DLL function

But they both show different values. I'm not sure if that's a correct way to inspect HWND... I'm not sure what type is HWND, integer, long?

Thanks for you help,
Alex

jkr

>>I'm not sure what type is HWND, integer, long?

A HWND is a void* - try to output the HWND's value this way:

WINLOGONDLL_API HWND SetWindow(HWND appWindow)
{
#ifdef _DEBUG
char scBuf [ 256];

wsprintf ( acBuf, "HWND: 0x%8.8x\n", hWnd);
OutputDebugString ( acBuf);
#endif

hWnd = appWindow;

return hWnd;
}

WINLOGONDLL_API UINT SendTestMessage()
{
#ifdef _DEBUG
char scBuf [ 256];

wsprintf ( acBuf, "HWND: 0x%8.8x\n", hWnd);
OutputDebugString ( acBuf);
#endif

UINT iMsg = RegisterWindowMessage(MY_MESSAGE_UNLOCK);
SendMessage(hWnd, iMsg, NULL, NULL);

return iMsg;
}

DebugView (http://www.sysinternals.com/ntw2k/freeware/debugview.shtml) is a great tool for displaying such debug messages.

astankovic

ASKER

Ok, that found the error. I think I'm missing some DLL fundamentals. Does each application that loads the DLL gets a separate "copy" of DLL? It seems that it does..
Because, i was setting hWnd from one application (C#), but calling SendTestMessage() from another (VB 6 for testing purposes)

So when I call SendTestMessage() from VB 6 app, the hWnd 0x00000000.
But when i call both SetWindow() and SendTestMessage() from C# app, hWnd has the same value in both functions.

So the question now i guess is how to set hWnd from one app, and use it in a call from another?

Thanks you for pointing me to DebugView, it is really great.

Alex

ASKER CERTIFIED SOLUTION

jkr

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jkr

BTW, see als http://support.microsoft.com/default.aspx?kbid=125677 ("HOWTO: Share Data Between Different Mappings of a DLL")