DropShadow for WPF Borderless Window

Question

I have a WPF Window with WindowStyle set to none. Is there some way I can force this window to drop a shadow (like the one you get when WindowStyle is not none)? I don't want to set AllowTransparency to true, because it affects the performance. And I also don't want to disable hardware rendering (I read somewhere that transparency performs better with it disabled).

Answer 1

I have written a little utility class that is able to do exactly what you want: drop a standard shadow over a borderless Window but having AllowsTransparency set to false.

You just have to call the DropShadowToWindow(Window window) method. It is preferred that you make this call just after the window's constructor's InitializeComponent(), but it will work even if you call it after the window is shown.

using System;
using System.Drawing.Printing;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Interop;

public static class DwmDropShadow
{
    [DllImport("dwmapi.dll", PreserveSig = true)]
    private static extern int DwmSetWindowAttribute(IntPtr hwnd, int attr, ref int attrValue, int attrSize);

    [DllImport("dwmapi.dll")]
    private static extern int DwmExtendFrameIntoClientArea(IntPtr hWnd, ref Margins pMarInset);

    /// <summary>
    /// Drops a standard shadow to a WPF Window, even if the window is borderless. Only works with DWM (Windows Vista or newer).
    /// This method is much more efficient than setting AllowsTransparency to true and using the DropShadow effect,
    /// as AllowsTransparency involves a huge performance issue (hardware acceleration is turned off for all the window).
    /// </summary>
    /// <param name="window">Window to which the shadow will be applied</param>
    public static void DropShadowToWindow(Window window)
    {
        if (!DropShadow(window))
        {
            window.SourceInitialized += new EventHandler(window_SourceInitialized);
        }
    }

    private static void window_SourceInitialized(object sender, EventArgs e)
    {
        Window window = (Window)sender;

        DropShadow(window);

        window.SourceInitialized -= new EventHandler(window_SourceInitialized);
    }

    /// <summary>
    /// The actual method that makes API calls to drop the shadow to the window
    /// </summary>
    /// <param name="window">Window to which the shadow will be applied</param>
    /// <returns>True if the method succeeded, false if not</returns>
    private static bool DropShadow(Window window)
    {
        try
        {
            WindowInteropHelper helper = new WindowInteropHelper(window);
            int val = 2;
            int ret1 = DwmSetWindowAttribute(helper.Handle, 2, ref val, 4);

            if (ret1 == 0)
            {
                Margins m = new Margins { Bottom = 0, Left = 0, Right = 0, Top = 0 };
                int ret2 = DwmExtendFrameIntoClientArea(helper.Handle, ref m);
                return ret2 == 0;
            }
            else
            {
                return false;
            }
        }
        catch (Exception ex)
        {
            // Probably dwmapi.dll not found (incompatible OS)
            return false;
        }
    }
}