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_SYSTEM_REQUIRED  = 0x00000001,
   ES_DISPLAY_REQUIRED = 0x00000002,
   // Legacy flag, should not be used.
   // ES_USER_PRESENT   = 0x00000004,
   ES_CONTINUOUS       = 0x80000000,
}

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);
    }

Alternative Managed API:

Do you know one? Please contribute it!