# C# <s>175</s> 171

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    class P{static void Main(){SendMessage(65535,274,61808,2);}[System.Runtime.InteropServices.DllImport("user32.dll")]static extern int SendMessage(int a,int b,int c,int d);}


A lot of the answers here don't actually make the screen *black*: on an LCD screen, the backlight remains on and bleeds through, leaving you with a darkish grey.

This little snippet actually tells Windows to turn off the screen, the same as the power off on inactivity.

Move the mouse or press a key to turn the screen back on.

Monitor power off adapted from http://stackoverflow.com/a/713519/1030702