TransparentTaskbar

using System;
using System.Runtime.InteropServices;

public class TransparentTaskbar {
    static class Interop {
        [DllImport("user32.dll", SetLastError = true)]
        internal static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

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

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

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

        [Flags]
        internal enum AccentFlags {
            // ...
            DrawLeftBorder = 0x20,
            DrawTopBorder = 0x40,
            DrawRightBorder = 0x80,
            DrawBottomBorder = 0x100,
            DrawAllBorders = (DrawLeftBorder | DrawTopBorder | DrawRightBorder | DrawBottomBorder)
            // ...
        }

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

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

    public static void Main(string[] CommandLine) {
        var accent = new Interop.AccentPolicy();
        accent.AccentState = Interop.AccentState.ACCENT_ENABLE_BLURBEHIND;

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

        var data = new Interop.WindowCompositionAttributeData();
        data.Attribute = Interop.WindowCompositionAttribute.WCA_ACCENT_POLICY;
        data.SizeOfData = Marshal.SizeOf(accent);
        data.Data = accentPtr;

        Interop.SetWindowCompositionAttribute(Interop.FindWindow("Shell_TrayWnd", null), ref data);
    }
}