Calling GetGUIThreadInfo via P/Invoke

I want to send keyboard input to a window in another process, without bringing that window to the foreground. I can use PostMessage to fake the WM_KEYDOWN and WM_KEYUP; all I need to know is which window handle should receive the keyboard input -- i.e., something like GetFocus, but for another, non-active application.

The GetGUIThreadInfo API looks promising -- it returns an hwndFocus for another app. But I've had no luck getting it to work from C# on my 64-bit OS. I've copied (and then further tweaked) the declarations from pinvoke.net, but all I ever get back is a generic error code (more details below).

I am setting cbSize before I call GetGUIThreadInfo, so I've avoided the most obvious potential problem.

I'm running 64-bit Vista, so I don't know whether the problem is that I'm not using the API correctly, or that it works differently in 64-bit -- I have yet to find a code sample that specifically says it works successfully in Win64.

Here's sample code. I'm using GetWindowThreadProcessId as recommended, so I don't think the problem has to do with mixing thread IDs with thread handles:

[StructLayout(LayoutKind.Sequential)]
internal struct Rect
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;
}

[StructLayout(LayoutKind.Sequential)]
internal class GuiThreadInfo
{
    public int cbSize;
    public uint flags;
    public IntPtr hwndActive;
    public IntPtr hwndFocus;
    public IntPtr hwndCapture;
    public IntPtr hwndMenuOwner;
    public IntPtr hwndMoveSize;
    public IntPtr hwndCaret;
    public Rect rcCaret