Question

I have a business requirement that for message boxes, the user cannot press the enter key to accept the default option, but has to press the key of the option. eg. Given a MessageBox with the options Yes/No, the user must press the Y or N keys. Now I’ve implemented this below using keyboard hooks, but when the code returns, the KeyUp event also gets returned to the calling code as well.

So the question is: How do I flush all the keyboard events before returning to the calling code?

I’ve removed boiler plate code, but if you need it, please advise.

The calling code:

    private static ResultMsgBox MsgResultBaseNoEnter(string msg, string caption, uint options)
    {
        ResultMsgBox res;
        _hookID = SetHook(_proc);
        try
        {
            res = MessageBox(GetForegroundWindow(), msg, caption, options);
        }
        finally
        {
            UnhookWindowsHookEx(_hookID);
        }
        return res;
    }

And the Hook Code:

    private static IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
    {
        if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
        {
            int vkCode = Marshal.ReadInt32(lParam);
            if (vkCode == VK_RETURN)
                return (IntPtr)(-1);
        }
        return CallNextHookEx(_hookID, nCode, wParam, lParam);
    }

Answer 1

Add these lines of code somewhere in your class (or in some static class that can be used by other classes):

[StructLayout(LayoutKind.Sequential)]
public class MSG
{
    public IntPtr hwnd;
    public uint message;
    public IntPtr wParam;
    public IntPtr lParam;
    public uint time;
    int x;
    int y;
}

[DllImport("user32")]
public static extern bool PeekMessage([Out]MSG lpMsg, IntPtr hWnd, uint wMsgFilterMin, uint wMsgFilterMax, int wRemoveMsg);

/// <summary>
/// Examines the message queue for key messages.
/// </summary>
/// <param name="remove">If this parameter is true, the returned message is also removed from the queue.</param>
/// <returns>Returns the next available key message, or null if there is no key message available.</returns>
public static MSG PeekKeyMessage(bool remove)
{
    MSG msg = new MSG();
    if (PeekMessage(msg, IntPtr.Zero, 0x0100 /*WM_KEYFIRST*/, 0x0109 /*WM_KEYLAST*/, remove ? 1 : 0))
        return msg;
    return null;
}

public static void RemoveAllKeyMessages()
{
    while (PeekKeyMessage(true) != null) ; // Empty body. Every call to the method removes one key message.
}

Calling RemoveAllKeyMessages() does exactly what you want.