SendInput

public static class WinAPI {
    public const uint MAPVK_VK_TO_SC = 0;
    public const uint MAPVK_SC_TO_VK = 1;

    [DllImport("user32.dll")]
    public static extern uint MapVirtualKey(uint uCode, uint uMapType);

    [DllImport("user32.dll")]
    public static extern uint SendInput(uint nInputs, [MarshalAs(UnmanagedType.LPArray), In] Input[] pInputs, int cbSize);

    [DllImport("user32.dll")]
    public static extern IntPtr GetMessageExtraInfo();

    public const int INPUT_MOUSE    = 0;
    public const int INPUT_KEYBOARD = 1;
    public const int INPUT_HARDWARE = 2;

    [StructLayout(LayoutKind.Explicit)]
    public struct Input {
        [FieldOffset(0)]
        public int Type;

        [FieldOffset(4)]
        public KeyboardInput Ki;

        [FieldOffset(4)]
        public MouseInput Mi;

        [FieldOffset(4)]
        public HardwareInput Hi;

        public static Input Keyboard(KeyboardInput input) {
            return new Input { Type = INPUT_KEYBOARD, Ki = input };
        }

        public static Input Mouse(MouseInput input) {
            return new Input { Type = INPUT_MOUSE, Mi = input };
        }

        public static Input Hardware(HardwareInput input) {
            return new Input { Type = INPUT_HARDWARE, Hi = input };
        }
    }

    public const uint KEYEVENTF_EXTENDED = 0x0001;
    public const uint KEYEVENTF_KEYUP    = 0x0002;
    public const uint KEYEVENTF_SCANCODE = 0x0008;

    public struct KeyboardInput {
        public ushort Vk;
        public ushort Scan;
        public uint Flags;
        public uint Time;
        public IntPtr ExtraInfo;

        public KeyboardInput(ushort Vk = 0, ushort Scan = 0, uint Flags = 0, uint Time = 0) {
            this.Vk = Vk;
            this.Scan = Scan;
            this.Flags = Flags;
            this.Time = Time;
            ExtraInfo = GetMessageExtraInfo();
        }
    }

    public struct MouseInput {
        public int Dx;
        public int Dy;
        public uint Data;
        public uint Flags;
        public uint Time;
        public IntPtr ExtraInfo;

        public MouseInput(int Dx = 0, int Dy = 0, uint Data = 0, uint Flags = 0, uint Time = 0) {
            this.Dx = Dx;
            this.Dy = Dy;
            this.Data = Data;
            this.Flags = Flags;
            this.Time = Time;
            ExtraInfo = GetMessageExtraInfo();
        }
    }

    public struct HardwareInput {
        public uint Msg;
        public ushort ParamL;
        public ushort ParamH;

        public HardwareInput(uint Msg = 0, ushort ParamL = 0, ushort ParamH = 0) {
            this.Msg = Msg;
            this.ParamL = ParamL;
            this.ParamH = ParamH;
        }
    }

    public static uint SendInput(Input[] inputs) {
        return SendInput((uint)inputs.Length, inputs, Marshal.SizeOf<Input>());
    }
}

public class Utils {
    public static void SendKeyEvent(uint keyCode, bool isDown) {
        uint flags = WinAPI.KEYEVENTF_SCANCODE;

        if(!isDown) flags |= WinAPI.KEYEVENTF_KEYUP;
        if((keyCode & 0x100) > 0) flags |= WinAPI.KEYEVENTF_EXTENDED;

        ushort scan = (ushort)WinAPI.MapVirtualKey(keyCode & 0xFF, WinAPI.MAPVK_VK_TO_SC);

        WinAPI.SendInput(new WinAPI.Input[] {
            WinAPI.Input.Keyboard(new WinAPI.KeyboardInput(Scan: scan, Flags: flags)),
        });
    }
}