Ever wanted to known how the Windows XP powertoy TaskSwitch.exe (available here http://www.microsoft.com/windowsxp/pro/downloads/powertoys.asp) does the window preview trick? It uses a new to Windows XP and Windows .Net server API called PrintWindow. Perf is not that great though...

The following C# code does the same. I have not copied the full interop P/Invoke code, if you're interested, drop me a note.

public static void PreviewWindow(
 IntPtr previewWindowHandle,
 int destinationX,
 int destinationY,
 int destinationWidth,
 int destinationHeight,
 IntPtr targetDeviceContext)
{
 // Take a snapshot of the window hwnd, stored in the memory device context hdcMem
 IntPtr hdc = GetWindowDC(previewWindowHandle);
 if (hdc != IntPtr.Zero)
 {
  IntPtr hdcMem = CreateCompatibleDC(hdc);
  if (hdcMem != IntPtr.Zero)
  {
   // get target window size
   RECT rc;
   GetWindowRect(previewWindowHandle, out rc);

   IntPtr hbitmap = CreateCompatibleBitmap(
    hdc,
    rc.right - rc.left,
    rc.bottom - rc.top);
   if (hbitmap != IntPtr.Zero)
   {
    SelectObject(hdcMem, hbitmap);

    // Do the magic
    PrintWindow(previewWindowHandle, hdcMem, 0);

    // Copy bits
    StretchBlt(
     targetDeviceContext,
     destinationX,
     destinationY,
     destinationWidth,
     destinationHeight,
     hdcMem,
     0,
     0,
     rc.right - rc.left,
     rc.bottom - rc.top,
     SRCCOPY);

    DeleteObject(hbitmap);
   }
   DeleteObject(hdcMem);
  }
  ReleaseDC(previewWindowHandle, hdc);
 }
}

// this is how you can use it in Winform code (beware the perf and add some error checks!)
protected override void OnPaint(PaintEventArgs pe)
{
 Graphics g = pe.Graphics;
 IntPtr hdc = g.GetHdc();

 // previewhandle is a choosen window in the client desktop
 Native.Preview(_previewHandle, 0, 0,  ClientSize.Width, ClientSize.Height, hdc);

 // cleanup
 g.ReleaseHdc(hdc);
}