WINAPI/DWMAPI Blur-behind window with irregular shape

Question

NB: THIS IS NOT A QUESTION ABOUT A BORDERLESS WINDOW.

So, I stumbled upon this program while I was exploring my Start menu the other day on Windows 7:

Answer 1

Here's a quickly hacked together WPF solution. It uses the hRgnBlur of the DWM_BLURBEHIND structure, and some interop.

This example will apply an ellipse-shaped background blur on the window.

You can easily convert this to an attached property or behavior for MVVM-friendliness. It's also a good idea to listen to the WM_DWMCOMPOSITIONCHANGED message and reapply the blur if needed.

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        WindowStyle = WindowStyle.None;
        AllowsTransparency = true;

        SourceInitialized += OnSourceInitialized;
    }

    private void OnSourceInitialized(object sender, EventArgs eventArgs)
    {
        if (!NativeMethods.DwmIsCompositionEnabled())
            return;

        var hwnd = new WindowInteropHelper(this).Handle;

        var hwndSource = HwndSource.FromHwnd(hwnd);
        var sizeFactor = hwndSource.CompositionTarget.TransformToDevice.Transform(new Vector(1.0, 1.0));

        Background = System.Windows.Media.Brushes.Transparent;
        hwndSource.CompositionTarget.BackgroundColor = Colors.Transparent;

        using (var path = new GraphicsPath())
        {
            path.AddEllipse(0, 0, (int)(ActualWidth * sizeFactor.X), (int)(ActualHeight * sizeFactor.Y));

            using (var region = new Region(path))
            using (var graphics = Graphics.FromHwnd(hwnd))
            {
                var hRgn = region.GetHrgn(graphics);

                var blur = new NativeMethods.DWM_BLURBEHIND
                {
                    dwFlags = NativeMethods.DWM_BB.DWM_BB_ENABLE | NativeMethods.DWM_BB.DWM_BB_BLURREGION | NativeMethods.DWM_BB.DWM_BB_TRANSITIONONMAXIMIZED,
                    fEnable = true,
                    hRgnBlur = hRgn,
                    fTransitionOnMaximized = true
                };

                NativeMethods.DwmEnableBlurBehindWindow(hwnd, ref blur);

                region.ReleaseHrgn(hRgn);
            }
        }
    }

    [SuppressUnmanagedCodeSecurity]
    private static class NativeMethods
    {
        [StructLayout(LayoutKind.Sequential)]
        public struct DWM_BLURBEHIND
        {
            public DWM_BB dwFlags;
            public bool fEnable;
            public IntPtr hRgnBlur;
            public bool fTransitionOnMaximized;
        }

        [Flags]
        public enum DWM_BB
        {
            DWM_BB_ENABLE = 1,
            DWM_BB_BLURREGION = 2,
            DWM_BB_TRANSITIONONMAXIMIZED = 4
        }

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern bool DwmIsCompositionEnabled();

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern void DwmEnableBlurBehindWindow(IntPtr hwnd, ref DWM_BLURBEHIND blurBehind);
    }
}

Used with the following XAML:

The result is: