Override The WindowProc Of A Different Process?

Question

Simply: I want to evaluate messages that are send to a window by WinApi. This window is not part of my application.

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So when messages are send to the window of let's say notepad I want to read them and forward them to the actual window proc of the notepad process. What is the right approach ? I have this so far. But it seems not to work:

private static IntPtr hook = IntPtr.Zero;

private static IntPtr callWndProc(int code, IntPtr wParam, IntPtr lParam)
{
    Console.WriteLine("hello world");
    CWPSTRUCT a = new CWPSTRUCT();
    Marshal.PtrToStructure(lParam, a);
    Console.WriteLine(a.message);
    return Windows.CallNextHookEx(hook, code, wParam, lParam);
}

static void Main(string[] args)
{
    Process[] procs = Process.GetProcessesByName("notepad");
    if (procs.Length > 0)
    {
        hook = Windows.SetWindowsHookEx(WH_CALLWNDPROC, Marshal.GetFunctionPointerForDelegate((HookProc)callWndProc), procs[0].Handle, procs[0].Id);
        // last result: (126) The specified module could not be found.
        // hook == 0
    }
    Console.ReadKey();
}