'using System.Windows.Interop;' where is it?

Question

I saw this code for shadows around borderless windows but here i my problem. using System.Windows.Interop; is underlined and i cant find it in references. Also in public static void DropShadowToWindow(Window window) this Window is underlined so i guess its linked to 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 a borderess window.
    /// Only works with DWM (Vista and Seven).
    ///
    /// This is much more efficient than setting AllowsTransparency to true 
    /// and using the DropShadow effect, as AllowsTransparency will turn off
    /// acceleration for all the windows. (This is a huge performance issue.)
    /// </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;
        }
    }
}

Answer 1

It's in WindowsBase.DLL, which was introduced in .NET Framework 3.0. It is located in c:\Program Files\Reference Assemblies\Microsoft\Framework\v3.0\WindowsBase.dll

Answer 2

Just in case someone comes here to search for System.Windows.Interop.CompositionMode: it didn't make it to the final .Net 4.5 version.

See http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/2644120-bring-back-the-hwndhost-isredirected-and-compositi?page=2&per_page=20