Using a low-level keyboard hook to change keyboard characters

Question

I'm creating a custom keyboard layout. As the beginning step, I want to have the user press a key, have my keyboard hook intercept it, and output a different key of my choosing.

I found this keyboard hook code, which I'm trying to slightly modify for my purposes: http://blogs.msdn.com/toub/archive/2006/05/03/589423.aspx

I've changed the relevant method to this:

private static IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
{
    if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
    {
        KBDLLHOOKSTRUCT replacementKey = new KBDLLHOOKSTRUCT();
        Marshal.PtrToStructure(lParam, replacementKey);
        replacementKey.vkCode = 90; // char 'Z'
        Marshal.StructureToPtr(replacementKey, lParam, true);
    }
    return CallNextHookEx(_hookID, nCode, wParam, lParam);
}

I want it to declare a new KBD structure object, copy the KBD structure supplied by the keyboard hook into it, modify my object's vkCode to use a different character, and then overwrite the supplied object with my modified version. This should hopefully keep everything the same except for the fact that it writes a different character.

Unfortunately, it's not working. The original keyboard character is typed. The Visual Studio output pane also gets a A first chance exception of type 'System.ArgumentException' occurred in MirrorBoard.exe error.

What can I do here to intercept the keyboard hook and replace it with a character of my choosing?

Thanks!

Answer 1

The second parameter for Marshal.PtrToStructure must be a class not a struct and KBDLLHOOKSTRUCT is probably a struct.

Instead you should use it like this:

KBDLLHOOKSTRUCT replacementKey = (KBDLLHOOKSTRUCT)Marshal.PtrToStructure(lParam, typeof(KBDLLHOOKSTRUCT));
replacementKey.vkCode = 90; // char 'Z'
Marshal.StructureToPtr(replacementKey, lParam, false);