public partial class Form1 : Form
    {
        public int WM_SYSCOMMAND = 0x0112;
        public int SC_MONITORPOWER = 0xF170; 
        [DllImport("user32.dll")]
        private static extern int SendMessage(int hWnd, int hMsg, int wParam, int lParam);

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            SendMessage( this.Handle.ToInt32() , WM_SYSCOMMAND , SC_MONITORPOWER ,2 );//DLL function
        }
    }