KeyboardPressSample.cs

// キーコード
short key = 65;

INPUT[] inputs = new INPUT[] {
    new INPUT {
        type = NativeMethods.INPUT_KEYBOARD,
        ui = new INPUT_UNION {
            keyboard = new KEYBDINPUT {
                wVk = key,
                wScan = (short) NativeMethods.MapVirtualKey (key, 0),
                dwFlags = NativeMethods.KEYEVENTF_EXTENDEDKEY | NativeMethods.KEYEVENTF_KEYDOWN,    // キーの押し下げ
                dwExtraInfo = IntPtr.Zero,
                time = 0
            }
        }
    },
    new INPUT {
        type = NativeMethods.INPUT_KEYBOARD,
        ui = new INPUT_UNION {
            keyboard = new KEYBDINPUT {
                wVk = key,
                wScan = (short) NativeMethods.MapVirtualKey (key, 0),
                dwFlags = NativeMethods.KEYEVENTF_EXTENDEDKEY | NativeMethods.KEYEVENTF_KEYUP,      // キーの押し上げ
                dwExtraInfo = IntPtr.Zero,
                time = 0
            }
        }
    }
};

NativeMethods.SendInput (2, ref inputs[0], Marshal.SizeOf (inputs[0]));