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);
}
}

#include <windows.h>
#include "stdafx.h"

TCHAR szClassName[] = TEXT("Blur");

LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
   switch (msg)
   {
   case WM_NCHITTEST:
      wParam = DefWindowProc(hWnd, msg, wParam, lParam);
      if (wParam == HTCLIENT)
         return HTCAPTION;
      else
         return wParam;
   case WM_DESTROY:
      PostQuitMessage(0);
      break;
   default:
      return DefWindowProc(hWnd, msg, wParam, lParam);
   }
   return 0;
}

void SetWindowBlur(HWND hWnd)
{
   const HINSTANCE hModule = LoadLibrary(TEXT("user32.dll"));
   if (hModule)
   {
      struct ACCENTPOLICY
      {
         int nAccentState;
         int nFlags;
         int nColor;
         int nAnimationId;
      };
      struct WINCOMPATTRDATA
      {
         int nAttribute;
         PVOID pData;
         ULONG ulDataSize;
      };
      typedef BOOL(WINAPI*pSetWindowCompositionAttribute)(HWND, WINCOMPATTRDATA*);
      const pSetWindowCompositionAttribute SetWindowCompositionAttribute = (pSetWindowCompositionAttribute)GetProcAddress(hModule, "SetWindowCompositionAttribute");
      if (SetWindowCompositionAttribute)
      {
         ACCENTPOLICY policy = { 3, 0, 0, 0 };
         WINCOMPATTRDATA data = { 19, &policy, sizeof(ACCENTPOLICY) };
         SetWindowCompositionAttribute(hWnd, &data);
      }
      FreeLibrary(hModule);
   }
}

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPreInst, LPSTR pCmdLine, int nCmdShow)
{
   MSG msg;
   WNDCLASS wndclass = {
      0,
      WndProc,
      0,
      0,
      hInstance,
      0,
      LoadCursor(0, IDC_ARROW),
      (HBRUSH)GetStockObject(BLACK_BRUSH),
      0,
      szClassName
   };
   RegisterClass(&wndclass);
   RECT taskpos;
   HWND taskbar = FindWindow(L"Shell_TrayWnd", NULL);
   GetWindowRect(taskbar, &taskpos);
   HWND hWnd = CreateWindowEx(
      WS_EX_TOOLWINDOW | WS_EX_TOPMOST ,
      szClassName,
      TEXT("Blur"),
      WS_POPUP,
      taskpos.left,
      taskpos.top,
      taskpos.right - taskpos.left,
      taskpos.bottom - taskpos.top,
      0,
      0,
      hInstance,
      0
   );
   SetParent(taskbar, hWnd);
   SetWindowBlur(hWnd);
   ShowWindow(hWnd, SW_SHOW);
   // i wanna add some darkening effect later on, draw a half-transparent black rectangle on the whole window
   while (GetMessage(&msg, 0, 0, 0))
   {
      TranslateMessage(&msg);
      DispatchMessage(&msg);
   }
   return msg.wParam;
}