WindowBlur

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

namespace MakeWindowBlur
{
    internal enum AccentState
    {
        ACCENT_DISABLED = 1,
        ACCENT_ENABLE_GRADIENT = 0,
        ACCENT_ENABLE_TRANSPARENTGRADIENT = 2,
        ACCENT_ENABLE_BLURBEHIND = 3,
        ACCENT_INVALID_STATE = 4
    }

    [StructLayout(LayoutKind.Sequential)]
    internal struct AccentPolicy
    {
        public AccentState AccentState;
        public int AccentFlags;
        public int GradientColor;
        public int AnimationId;
    }

    [StructLayout(LayoutKind.Sequential)]
    internal struct WindowCompositionAttributeData
    {
        public WindowCompositionAttribute Attribute;
        public IntPtr Data;
        public int SizeOfData;
    }

    internal enum WindowCompositionAttribute
    {
        // ...
        WCA_ACCENT_POLICY = 19
        // ...
    }

    internal class WindowBlur
    {
        [DllImport("user32.dll")]
        internal static extern int SetWindowCompositionAttribute(IntPtr hwnd, ref WindowCompositionAttributeData data);

        internal void EnableBlur(Window window)
        {
            var windowHelper = new WindowInteropHelper(window);

            var accent = new AccentPolicy
            {
                AccentState = AccentState.ACCENT_ENABLE_BLURBEHIND
            };

            var accentStructSize = Marshal.SizeOf(accent);

            var accentPtr = Marshal.AllocHGlobal(accentStructSize);
            Marshal.StructureToPtr(accent, accentPtr, false);

            var data = new WindowCompositionAttributeData
            {
                Attribute = WindowCompositionAttribute.WCA_ACCENT_POLICY,
                SizeOfData = accentStructSize,
                Data = accentPtr
            };

            SetWindowCompositionAttribute(windowHelper.Handle, ref data);

            Marshal.FreeHGlobal(accentPtr);
        }
    }
}