WPF drop shadow with Windows DWM API

Ever since I started writing windows applications, even though most of them were not in C or C++, almost every time I would refer back to Charles Petzold’s Programming Windows. Whether it was Visual Basic, Delphi, .NET or even Java, there would eventually come a point, where the only way to achieve something would be to dig into the Win32 API and hook into the message loop directly.

I don’t know why I thought writing a WPF application in .NET 4 for Windows 7 would be any different.

If you start with a default empty window it is rendered nicely with a drop shadow around it (like all other windows).

However, I wanted to achieve an alternative layout, without the frame but with the shadow still on. Unfortunately setting WindowStyle="None" creates a rather blunt rectangle.

There is always the option to do something inside the client window to pretend your window actually does have a shadow but it is somewhat messy and just not as pretty (or perhaps I wasn’t ready to tinker with the Canvas and Effects enough). So will say it also comes with a performance penalty (since you would require transparent background).

Finally, with a little bit of Interop spice by calling Desktop Window Manager APIs I got the desired effect:

Mind you it only works if DWM is available (that is in Vista and Windows7) but I’m fine with that.

Here is the required code:

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

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

void ShellWindow_SourceInitialized(object sender, EventArgs e)
{
    var helper = new WindowInteropHelper(this);
    int val = 2;
    DwmSetWindowAttribute(helper.Handle, 2, ref val, 4);
    var m = new Margins
    {
        bottomHeight = -1,
        leftWidth = -1,
        rightWidth = -1,
        topHeight = -1
    };
    DwmExtendFrameIntoClientArea(helper.Handle, ref m);
}

If you want to dig deeper there is WPF Shell Integration Library that does some of this heavy lifting for you.

P.S. Seems like Java folks have similar challenges.