How to prevent Windows from entering idle state?

You’ve to use SetThreadExecutionState function. Something like this:

public partial class MyWinForm: Window
{
    private uint fPreviousExecutionState;

    public Window1()
    {
        InitializeComponent();

        // Set new state to prevent system sleep
        fPreviousExecutionState = NativeMethods.SetThreadExecutionState(
            NativeMethods.ES_CONTINUOUS | NativeMethods.ES_SYSTEM_REQUIRED);
        if (fPreviousExecutionState == 0)
        {
            Console.WriteLine("SetThreadExecutionState failed. Do something here...");
            Close();
        }
    }

    protected override void OnClosed(System.EventArgs e)
    {
        base.OnClosed(e);

        // Restore previous state
        if (NativeMethods.SetThreadExecutionState(fPreviousExecutionState) == 0)
        {
            // No way to recover; already exiting
        }
    }
}

internal static class NativeMethods
{
    // Import SetThreadExecutionState Win32 API and necessary flags
    [DllImport("kernel32.dll")]
    public static extern uint SetThreadExecutionState(uint esFlags);
    public const uint ES_CONTINUOUS = 0x80000000;
    public const uint ES_SYSTEM_REQUIRED = 0x00000001;
}