TurnoffMonitor/TurnoffMonitorOnLock/MonitorPowerApi/MonitorPower.cs

using System.Runtime.InteropServices;

namespace TurnoffMonitorOnLock.MonitorPowerApi
{
    internal static class MonitorPower
    {
        [DllImport("user32.dll")]
        private static extern int PostMessage(int hWnd, int hMsg, int wParam, int lParam);

        public static void SetMonitorInState(MonitorState state)
        {
            PostMessage(0xFFFF, 0x112, 0xF170, (int)state);
        }
    }
}