Tricky c# form keyboard input focus issue

Question

I have an application using the following code to get input based on a file generated from a third party application.

    protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
    {
        const int WM_KEYDOWN = 0x100;
        const int WM_SYSKEYDOWN = 0x104;

        if ((msg.Msg == WM_KEYDOWN) || (msg.Msg == WM_SYSKEYDOWN))
        {
            switch (keyData)
            {
                case Keys.F1:
                    clicked_F1(null, null);
                    break;

                case Keys.F2:
                    clicked_F2(null, null);
                    break;

                case Keys.F3:
                    clicked_F3(null, null);
                    break;
            }
            return true;
        }
        return base.ProcessCmdKey(ref msg, keyData);
    }

Form.TopMost is set to true and this.Activate() is called during the OnShown callback. This works on most machines but occasionally the keyboard input will not be passed to my application, windows help for example will pop up if F1 is pressed.

Can anyone explain this behavior? I need to ensure my form will receive these keyboard events.

Cheers, Richard

Answer 1

Having your form TopMost is not sufficient to ensure this method runs. Your form also needs to have the focus. That requires calling its Activate() method (SetForegroundWindow in the Win32 API). Which is not guaranteed to run, you cannot steal the focus away from a window that the user is actively using. Only when enough time has passed since the last input event can you grab the focus.

Not sure what you are really trying to do, maybe you need a keyboard hook or use RegisterHotKey.