IntPtr hwnd = new System.Windows.Interop.WindowInteropHelper(window).Handle;

window.Background = Brushes.Transparent;
            HwndSource.FromHwnd(hwnd).CompositionTarget.BackgroundColor = Colors.Transparent;

 DwmApi.DwmExtendFrameIntoClientArea(hwnd, margins);

 public partial class Window1 : System.Windows.Window
    {

        public Window1()
        {
            InitializeComponent();

        }

        protected override void OnSourceInitialized(EventArgs e)
        {
            base.OnSourceInitialized(e);
            DWMLib.AeroHelper.ExtendGlassFrame(this, new Thickness(-1));

        }

    }

public class AeroHelper
    {
        public static bool ExtendGlassFrame(Window window, Thickness margin)
        {
            if (!DwmApi.DwmIsCompositionEnabled())
                return false;

            IntPtr hwnd = new WindowInteropHelper(window).Handle;
            if (hwnd == IntPtr.Zero)
                throw new InvalidOperationException("The Window must be shown before extending glass.");

            // Set the background to transparent from both the WPF and Win32 perspectives
            window.Background = Brushes.Transparent;
            HwndSource.FromHwnd(hwnd).CompositionTarget.BackgroundColor = Colors.Transparent;

            DWMLib.DwmApi.MARGINS margins = new DWMLib.DwmApi.MARGINS((int)margin.Left, (int)margin.Top, (int)margin.Right, (int)margin.Bottom);
            DwmApi.DwmExtendFrameIntoClientArea(hwnd, margins);

            return true;
        }

public class DwmApi
    {
        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern void DwmEnableBlurBehindWindow(IntPtr hWnd, DWM_BLURBEHIND pBlurBehind);

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern void DwmExtendFrameIntoClientArea(IntPtr hWnd, MARGINS pMargins);

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

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern void DwmGetColorizationColor(
            out int pcrColorization,
            [MarshalAs(UnmanagedType.Bool)]out bool pfOpaqueBlend);

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

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern IntPtr DwmRegisterThumbnail(IntPtr dest, IntPtr source);

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern void DwmUnregisterThumbnail(IntPtr hThumbnail);

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern void DwmUpdateThumbnailProperties(IntPtr hThumbnail, DWM_THUMBNAIL_PROPERTIES props);

        [DllImport("dwmapi.dll", PreserveSig = false)]
        public static extern void DwmQueryThumbnailSourceSize(IntPtr hThumbnail, out Size size);

        [StructLayout(LayoutKind.Sequential)]
        public class DWM_THUMBNAIL_PROPERTIES
        {
            public uint dwFlags;
            public RECT rcDestination;
            public RECT rcSource;
            public byte opacity;
            [MarshalAs(UnmanagedType.Bool)]
            public bool fVisible;
            [MarshalAs(UnmanagedType.Bool)]
            public bool fSourceClientAreaOnly;

            public const uint DWM_TNP_RECTDESTINATION = 0x00000001;
            public const uint DWM_TNP_RECTSOURCE = 0x00000002;
            public const uint DWM_TNP_OPACITY = 0x00000004;
            public const uint DWM_TNP_VISIBLE = 0x00000008;
            public const uint DWM_TNP_SOURCECLIENTAREAONLY = 0x00000010;
        }

        [StructLayout(LayoutKind.Sequential)]
        public class MARGINS
        {
            public int cxLeftWidth, cxRightWidth, cyTopHeight, cyBottomHeight;

            public MARGINS(int left, int top, int right, int bottom)
            {
                cxLeftWidth = left;
                cyTopHeight = top;
                cxRightWidth = right;
                cyBottomHeight = bottom;
            }
        }

        [StructLayout(LayoutKind.Sequential)]
        public class DWM_BLURBEHIND
        {
            public uint dwFlags;
            [MarshalAs(UnmanagedType.Bool)]
            public bool fEnable;
            public IntPtr hRegionBlur;
            [MarshalAs(UnmanagedType.Bool)]
            public bool fTransitionOnMaximized;

            public const uint DWM_BB_ENABLE = 0x00000001;
            public const uint DWM_BB_BLURREGION = 0x00000002;
            public const uint DWM_BB_TRANSITIONONMAXIMIZED = 0x00000004;
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct RECT
        {
            public int left, top, right, bottom;

            public RECT(int left, int top, int right, int bottom)
            {
                this.left = left;
                this.top = top;
                this.right = right;
                this.bottom = bottom;
            }
        }
    }