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[DllImport("user32.dll")]
static extern void keybd_event(byte bVk, byte bScan, uint dwFlags,
   UIntPtr dwExtraInfo);

public enum KeyboardEventFlags {
KEYDOWN = 0,
KEYUP = 2
}					

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keybd_event(Keys.K, 0, KeyboardEventFlags.KEYDOWN, UIntPtr.Zero);					

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    class Program
    {
        [DllImport("user32.dll")]
        static extern void keybd_event(byte bVk, byte bScan, uint dwFlags,
           UIntPtr dwExtraInfo);

        [DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
        private static extern IntPtr GetForegroundWindow();

        [DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
        private static extern Int32 GetWindowThreadProcessId(IntPtr hWnd, out Int32 lpdwProcessId);

        public enum KeyboardEventFlags
        {
            KEYDOWN = 0,
            KEYUP = 2
        }

        static void Main(string[] args)
        {

            while (true)
            {
                var fgw = GetForegroundWindow();
                int procID;
                var fwd_procID = GetWindowThreadProcessId(fgw, out procID);
                var proc = Process.GetProcessById(procID);

                if(proc != null && proc.ProcessName == "notepad")
                    keybd_event((byte)Keys.K, 0, (int)KeyboardEventFlags.KEYDOWN, UIntPtr.Zero);

                Thread.Sleep(500);
            }
             
        }
    }