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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

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