Purpose: Mirror a window of an external process by capturing images from this window and presenting the images in a picturebox using a timer.

Problem: When my application is in focus, everything happens fine! But if I switch the focus to the other window (not from my application), the picturebox stops refreshing the images. When I focus on my application again, everything returns to normal.

Limitation: I need the picturebox to continue updating even though my application is in the background.

Timer to capture images and show in picturebox:
Code:
private void timer_picBOX_refresh_Tick(object sender, EventArgs e)
   {
      pictureBox1.BackgroundImage = PrintScreen.CaptureWindow(GAME_MainHandle);            
      pictureBox1.Refresh();            
   }
Class to capture specific window:
Code:
public class class_ScreenCapture
    {
        public Image CaptureScreen()
        {
            return CaptureWindow(User32.GetDesktopWindow());
        }
     
	    /// <summary>
        /// Creates an Image object containing a screen shot of a specific window
        /// </summary>
        public Image CaptureWindow(IntPtr handle, int imgX = 0, int imgY = 0, int largura = 0, int altura = 0)
        {
            // get te hDC of the target window
            IntPtr hdcSrc = User32.GetWindowDC(handle);
            // get the size
            User32.RECT windowRect = new User32.RECT();
            User32.GetWindowRect(handle, ref windowRect);

            if (largura == 0 || altura == 0)
            {
                largura = windowRect.right - windowRect.left;
                altura = windowRect.bottom - windowRect.top;
            }

            // create a device context we can copy to
            IntPtr hdcDest = GDI32.CreateCompatibleDC(hdcSrc);
            // create a bitmap we can copy it to,
            // using GetDeviceCaps to get the width/height
            IntPtr hBitmap = GDI32.CreateCompatibleBitmap(hdcSrc, largura, altura);
            // select the bitmap object
            IntPtr hOld = GDI32.SelectObject(hdcDest, hBitmap);
            // bitblt over
            GDI32.BitBlt(hdcDest, 0, 0, largura, altura, hdcSrc, imgX, imgY, GDI32.SRCCOPY);
            // restore selection
            GDI32.SelectObject(hdcDest, hOld);
            // clean up 
            GDI32.DeleteDC(hdcDest);
            User32.ReleaseDC(handle, hdcSrc);

            // get a .NET image object for it
            Image img = Image.FromHbitmap(hBitmap);
            // free up the Bitmap object
            GDI32.DeleteObject(hBitmap);

            return img;
        }        

        /// <summary>
        /// Helper class containing Gdi32 API functions
        /// </summary>
        private class GDI32
        {

            public const int SRCCOPY = 0x00CC0020; // BitBlt dwRop parameter

            [DllImport("gdi32.dll")]
            public static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest,
                int nWidth, int nHeight, IntPtr hObjectSource,
                int nXSrc, int nYSrc, int dwRop);
            [DllImport("gdi32.dll")]
            public static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth,
                int nHeight);
            [DllImport("gdi32.dll")]
            public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
            [DllImport("gdi32.dll")]
            public static extern bool DeleteDC(IntPtr hDC);
            [DllImport("gdi32.dll")]
            public static extern bool DeleteObject(IntPtr hObject);
            [DllImport("gdi32.dll")]
            public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
        }

        /// <summary>
        /// Helper class containing User32 API functions
        /// </summary>
        private class User32
        {
            [StructLayout(LayoutKind.Sequential)]
            public struct RECT
            {
                public int left;
                public int top;
                public int right;
                public int bottom;
            }

            [DllImport("user32.dll")]
            public static extern IntPtr GetDesktopWindow();
            [DllImport("user32.dll")]
            public static extern IntPtr GetWindowDC(IntPtr hWnd);
            [DllImport("user32.dll")]
            public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);
            [DllImport("user32.dll")]
            public static extern IntPtr GetWindowRect(IntPtr hWnd, ref RECT rect);

        }
    }