private const int WM_SYSCOMMAND = 0x0112;
private const int SC_MONITORPOWER = 0xF170;
private const int MONITOR_ON = -1;
private const int MONITOR_OFF = 2;
/// <summary>
/// 关闭显示器
/// </summary>
public static void Off()
{
    SendMessage(0xffff, WM_SYSCOMMAND, SC_MONITORPOWER, MONITOR_OFF);
}
/// <summary>
/// 关闭显示器
/// </summary>
public static void On()
{
    SendMessage(0xffff, WM_SYSCOMMAND, SC_MONITORPOWER, MONITOR_ON);
}

[DllImport("user32.dll")]
public static extern int SendMessage(int hWnd, int hMsg, int wParam, int lParam);