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Asked: 2026-05-11T18:02:03+00:00

I need to get the Handler to the child Window of a certain application

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I need to get the Handler to the child Window of a certain application that is running. I have the main window handler, but I need to know which specific child window is active, in order to use the SendMessage/PostMessage.

I finally managed to do this using the following code, using firefox:

    [DllImport("user32.dll")]
    static extern uint GetWindowThreadProcessId(IntPtr hWnd, IntPtr ProcessId);

    [DllImport("user32.dll", EntryPoint = "GetGUIThreadInfo")]
    internal static extern bool GetGUIThreadInfo(uint idThread, out GUITHREADINFO threadInfo);


    private void button1_Click(object sender, EventArgs e)
    {
        //start firefox
        firefox = new Process();
        firefox.StartInfo.FileName = @"C:\Program Files\Mozilla Firefox\firefox.exe";
        firefox.Start();
        Thread.Sleep(10000);

        // get thread of the main window handle of the process
        var threadId = GetWindowThreadProcessId(firefox.MainWindowHandle, IntPtr.Zero);

        // get gui info
        var info = new GUITHREADINFO();
        info.cbSize = (uint)Marshal.SizeOf(info);
        if (!GetGUIThreadInfo(threadId, out info))
            throw new Win32Exception();


        // send the letter W to the active window
        PostMessage(info.hwndActive, WM_KEYDOWN, (IntPtr)Keys.W, IntPtr.Zero);

    }

This works very well! However, if the application is not active, for example, if notepad is covering firefox, the GUIThreadInfo comes with every member null. Only if firefox is the top-most (active) application of windows, will the structure be filled.

I know this could be fixed by bringing firefox to the foreground but I needed to avoid doing this. Does anyone have any other idea to get the active child window of an application that is not the top-most window in Windows?

Thanks

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1 Answer

  1. Editorial Team

    If you have the topmost window handle for the process, you should be able to use GetTopWindow to receive the window at the top of the Z order. This should be the window that would be active if the application were set to be the active/current app.

    Edit:

    What about using AttachThreadInput to attach your thread to the other process thread?

    Once you’ve done that, GetFocus() and PostMessage()/SendMessage() should work. Just make sure you detach the input when you’re done.

    The only sample I can find of this is unfortunately in Delphi, but would be easy to translate.

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