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...

For Windows 10 (tested on Pro 64 bits), I was able to turn off the monitor using the SendMessage() technique mentioned in this page. However, impossible for me to turn the monitor back on: the "mouse move" trick did not work, the SendMessage() technique would turn the screen back on for one second then back off and using PostMessage() did not change anything.

But the trick is in fact really simple, all I had to do was simulate a keypress with SendKeys(). I'm using ALT here because in itself it has no impact on the system but it could be any other key.

SendKeys.SendWait("%");

If you're not using Windows.Forms, sending "ALT" also works using SendInput() but it's longer to implement.