How to stop the screen from going to sleep using C#?

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You can prevent the screen from going to sleep in C# by calling the SetThreadExecutionState method from kernel32.dll. Here's an example:

using System.Runtime.InteropServices;

[DllImport("kernel32.dll")]
static extern uint SetThreadExecutionState(uint esFlags);

const uint ES_CONTINUOUS = 0x80000000;
const uint ES_SYSTEM_REQUIRED = 0x00000001;

// Prevent screen from going to sleep
SetThreadExecutionState(ES_CONTINUOUS | ES_SYSTEM_REQUIRED);

// Allow screen to go to sleep again
SetThreadExecutionState(ES_CONTINUOUS);

The first call to SetThreadExecutionState with the ESCONTINUOUS and ESSYSTEMREQUIRED flags will prevent the screen from going to sleep. The second call to SetThreadExecutionState with just the ESCONTINUOUS flag will allow the screen to go to sleep again.