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Taking a screen shot of a window using Delphi code is rather easy.

A screen shot (screen capture) is a copy of the screen's contents that can be saved as a graphics file or displayed in a graphics "aware" control, for example TImage.

In most cases you will want to take a screen shot of the active window or the Windows Desktop.

What if you need to do a screen capture of all the running applications - most of them will be inactive and not visible to the user?

Windows XP introduces the new printing API, PrintWindow. This API enables the caller to snapshot a visual copy of a window into a device context.

Drop a TImage (named Image1) on a form and use the following code:

    WindowSnap(Self.Handle, Image1.Picture.Bitmap) ;
    Image1.Refresh;

The actual WindowSnap function is defined as:

    function WindowSnap(windowHandle: HWND; bmp: TBitmap): boolean;
    var
      r: TRect;
      user32DLLHandle: THandle;
      printWindowAPI: function(sourceHandle: HWND; destinationHandle: HDC;
        nFlags: UINT): BOOL; stdcall;
    begin
      result := False;
      user32DLLHandle := GetModuleHandle(user32) ;
      if user32DLLHandle <> 0 then
      begin
        @printWindowAPI := GetProcAddress(huser32, 'PrintWindow') ;
        if @printWindowAPI <> nil then
        begin
          GetWindowRect(windowHandle, r) ;
          bmp.Width := r.Right - r.Left;
          bmp.Height := r.Bottom - r.Top;
          bmp.Canvas.Lock;
          try
            result := printWindowAPI(windowHandle, bmp.Canvas.Handle, 0) ;
          finally
            bmp.Canvas.Unlock;
          end;
        end;
      end;
    end; (*WindowSnap*)

Note that the first parameter to the WindowSnap function is a HWND value (THandle) - the handle of the window you want to capture.