'mit System.Windows.Interop; " wo ist es?

Sah ich diesen code für Schatten rund um borderless windows, aber hier habe ich mein problem. using System.Windows.Interop; ist Unterstrichen und ich kann nicht finden Sie in den Referenzen. Auch in public static void DropShadowToWindow(Window window) dieses Fenster Unterstrichen ist, so dass ich denke, es ist verbunden mit Interop...

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

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 isborderless. Only works with DWM (Vista and Seven).
    ///This method is much more efficient than setting AllowsTransparency to true and using the DropShadow effect,
    ///as AllowsTransparency involves a huge permormance 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) //fixed typo
    {
        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;
        }
    }
}