C# Signature:

[DllImport("kernel32.dll", CharSet = CharSet.Auto,SetLastError = true)]
static extern EXECUTION_STATE SetThreadExecutionState(EXECUTION_STATE esFlags);

User-Defined Types:

FlagsAttribute

public enum EXECUTION_STATE :uint
{
     ES_AWAYMODE_REQUIRED = 0x00000040,
     ES_CONTINUOUS = 0x80000000,
     ES_DISPLAY_REQUIRED = 0x00000002,
     ES_SYSTEM_REQUIRED = 0x00000001
     // Legacy flag, should not be used.
     // ES_USER_PRESENT = 0x00000004
}

Notes:

There is no need to store the state you set, Windows remembers it for you. Just set it back to ES_CONTINUOUS when you don't want it anymore.

Also note that this setting is per thread/application not global, so if you go to ES_CONTINUOUS and another app/thread is still setting ES_DISPLAY the   display will be kept on.

Note that the return value is the EXECUTION_STATE that ''was'' set.

Tips & Tricks:

Description of what the different EXECUTION_STATE does

http://msdn.microsoft.com/en-us/library/aa373208(v=vs.85).aspx

Sample Code:

    void PreventMonitorPowerdown ()
    {
        SetThreadExecutionState(EXECUTION_STATE.ES_DISPLAY_REQUIRED | EXECUTION_STATE.ES_CONTINUOUS);
    }

    void AllowMonitorPowerdown ()
    {
        SetThreadExecutionState(EXECUTION_STATE.ES_CONTINUOUS);
    }

    void PreventSleep ()
    {
        // Prevent Idle-to-Sleep (monitor not affected)
        SetThreadExecutionState(EXECUTION_STATE.ES_CONTINUOUS | EXECUTION_STATE.ES_AWAYMODE_REQUIRED);
    }

    void KeepSystemAwake ()
    {
        SetThreadExecutionState(EXECUTION_STATE.ES_SYSTEM_REQUIRED);
    }

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