Turn on/off monitor

Did you even try googling it?

First hit: http://www.codeproject.com/KB/cs/Monitor_management_guide.aspx

I am not surprised you need to use some DLL's supplied by Windows.

(I guessed you needed a C# solution, because that's the only tag you applied).

EDIT February 8th 2013:

It was mentioned that the solution no longer worked under Windows 7 en 8. Well here is one that works nicely under Windows 7, haven't tried Windows 8 yet.

http://cocoa.ninja/posts/Turn-off-your-monitor-in-Csharp.html

namespace MonitorOff {

    public enum MonitorState {
        MonitorStateOn = -1,
        MonitorStateOff = 2,
        MonitorStateStandBy = 1
    }

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

        public Form1() {
            InitializeComponent();
            SystemEvents.SessionSwitch += SystemEvents_SessionSwitch;
        }

        void SystemEvents_SessionSwitch(object sender, SessionSwitchEventArgs e) {
            SetMonitorInState(MonitorState.MonitorStateOff);
        }

        private void button1_Click(object sender, EventArgs e) {
            SetMonitorInState(MonitorState.MonitorStateOff);
        }

        private void SetMonitorInState(MonitorState state) {
            SendMessage(0xFFFF, 0x112, 0xF170, (int)state);
        }
    }
}

Press the on/off button

If you want to do it in code, apparently this is possible in the Win32 API:

SendMessage hWnd, WM_SYSCOMMAND, SC_MONITORPOWER, param

where WM_SYSCOMMAND = 0x112 and SC_MONITORPOWER = 0xF170 and param indicates the mode to put the monitor in: -1 : on 2 : off 1 : energy saving mode

hWnd can be a handle for any window - so if you have a Form, something like this should work

int WM_SYSCOMMAND = 0x112;
int SC_MONITORPOWER = 0xF170;

[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SendMessage(IntPtr hWnd, int wMsg, IntPtr wParam, IntPtr lParam);

public static void Main(string[] args)
{
    Form f = new Form();
    bool turnOff = true;   //set true if you want to turn off, false if on
    SendMessage(f.Handle, WM_SYSCOMMAND, (IntPtr)SC_MONITORPOWER, (IntPtr)(turnOff ? 2 : -1));
}

Note I haven't actually tried this...

The answer https://stackoverflow.com/a/713504/636189 above works great for turning off a Windows 7/8 monitor but not for waking it up. On those systems you'll need to do something hackish like this (as found https://stackoverflow.com/a/14171736/636189):

[DllImport("user32.dll")]
static extern void mouse_event(Int32 dwFlags, Int32 dx, Int32 dy, Int32 dwData, UIntPtr dwExtraInfo);

private const int MOUSEEVENTF_MOVE = 0x0001;

private void Wake(){
mouse_event(MOUSEEVENTF_MOVE, 0, 1, 0, UIntPtr.Zero);
Sleep(40);
mouse_event(MOUSEEVENTF_MOVE, 0, -1, 0, UIntPtr.Zero);
}

For who wants this functionality on a console application:

using System;
using System.Runtime.InteropServices;
using System.Timers;

namespace TurnScreenOFF
{
    class Program
    {
        private static int WM_SYSCOMMAND = 0x0112;
        private static uint SC_MONITORPOWER = 0xF170;

        public static void Main(string[] args)
        {
            SendMessage(GetConsoleWindow(), WM_SYSCOMMAND, (IntPtr)SC_MONITORPOWER, (IntPtr)2);
        }

        [DllImport("kernel32.dll")]
        static extern IntPtr GetConsoleWindow();

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        private static extern int SendMessage(IntPtr hWnd, int wMsg, IntPtr wParam, IntPtr lParam);
    }
}

Adaptated and tested. 100% working on Windows 8.

This code can be useful for turning on and turning off.. It worked in Windows 7 also.

   private int SC_MONITORPOWER = 0xF170;

    private uint WM_SYSCOMMAND = 0x0112;

    [DllImport("user32.dll")]
    static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);



    enum MonitorState
    {
        ON = -1,
        OFF = 2,
        STANDBY = 1
    }
    private void SetMonitorState(MonitorState state)
    {
        Form frm = new Form();

        SendMessage(frm.Handle, WM_SYSCOMMAND, (IntPtr)SC_MONITORPOWER, (IntPtr)state);

    }

For calling the function you must do like:

SetMonitorState(MonitorState.ON);

OR

SetMonitorState(MonitorState.OFF);

Note: This code tested in WPF Application. With the below namespaces:

using System.Runtime.InteropServices;
using System.Windows.Forms;