Detecting which screen a fullscreen app occupies

How to correctly detect the screen that a fullscreen capable application is active on.

Programming by Sverrir Sigmundarson

Detecting which screen an app is running on in a multi-monitor setup can be a little tricky. Luckily there are a few handy utility functions available for you in the .NET framework under System.Windows.Forms.Screen.

However for applications that support fullscreen modes detecting the screen require a little extra trick as to achieve fullscreen the window bounds are made to actually extend the screen bounds by a little.

Differences between windowed and fullscreen modes

Differences between windowed and fullscreen modes

The code below handles this correctly:

public Screen DetectScreen(IntPtr windowHandle)
{
    // Figure out which screen the window is located on (in a multi screen setup)

    var appWindowBounds = GetWindowBounds(windowHandle);
    foreach (var screen in Screen.AllScreens)
    {
        // If the app is not fullscreen then the screen.bounds will contain the window

        // if the app is fullscreen then IT will actually contain the screen bounds

        if (screen.Bounds.Contains(appWindowBounds) ||
            appWindowBounds.Contains(screen.Bounds))
        {
            return screen;
        }
    }

    // By default use the primary screen

    return Screen.PrimaryScreen;
}

For completeness below is the implementation for the GetWindowBounds function.

[DllImport("user32.dll", SetLastError = true)]
[return: MarshalAs(UnmanagedType.Bool)]
internal static extern bool GetWindowRect(IntPtr hwnd, out RECT lpRect);

[StructLayout(LayoutKind.Sequential)]
internal struct RECT
{
    public int Left;        // x position of upper-left corner

    public int Top;         // y position of upper-left corner

    public int Right;       // x position of lower-right corner

    public int Bottom;      // y position of lower-right corner

}

private Rectangle GetWindowBounds(IntPtr windowHandle)
{
    Win32.RECT rct;
    if (!Win32.GetWindowRect(windowHandle, out rct))
    {
        return Rectangle.Empty;
    }
    return rct.ToRectangle();
}

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