Prevent Windows workstation (desktop) from locking while running a WPF program

The solution has been pointed out through the comments, but I’m providing a simple starter solution for anyone else arriving via a web search:

/// <summary>
/// Interaction logic for App.xaml
/// </summary>
public partial class App : Application
{
    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern EXECUTION_STATE SetThreadExecutionState(EXECUTION_STATE esFlags);

    public App()
    {
        InitializeComponent();

        App.Current.Startup += new StartupEventHandler((sender, e) =>
            {
                SetThreadExecutionState(EXECUTION_STATE.ES_DISPLAY_REQUIRED | EXECUTION_STATE.ES_CONTINUOUS);
            });
        App.Current.Exit += new ExitEventHandler((sender, e) =>
            {
                SetThreadExecutionState(EXECUTION_STATE.ES_CONTINUOUS);
            });
    }
}

[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
}

An alternative place to put the logic would be within an event handler for StateChanged on your main application window:

this.StateChanged += new EventHandler((sender, e) =>
    {
        if (WindowState == System.Windows.WindowState.Maximized)
        {
            SetThreadExecutionState(EXECUTION_STATE.ES_DISPLAY_REQUIRED | EXECUTION_STATE.ES_CONTINUOUS);
        }
        else
        {
            SetThreadExecutionState(EXECUTION_STATE.ES_CONTINUOUS);
        }
    });