Screen Capture on Windows Phone 7

In some instances when working on projects we would require to take a screen capture of the current screen the user is working on.  I regularly did this on .NET Compact Framework 3.5 (pre Windows Phone 7) and is a little bit of mess because of all the PInvokes involved. 

I decided to create a quick sample app that captures the screen on both platforms to compare the code.  The sample app basically just places the captured screen inside a pictureBox control (NETCF 3.5) or Image control (Silverlight for WP7).  The outer PictureBox/Image control just displays some standard images that come with Windows 7.

Compact Framework 3.5		Silverlight (Windows Phone 7)

Here is a comparison of capture an image of the current screen on Windows Phone 7 using Silverlight and .NET Compact Framework

.NET Compact Framework

private Bitmap CaptureScreen()
{
    Bitmap b = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
    using (Graphics g = Graphics.FromImage(b))
    {
        IntPtr hdcSrc = IntPtr.Zero;
        IntPtr hdcDest = g.GetHdc();
        try 
        {
            //get the entire window by passing in IntPtr.Zero
            hdcSrc = GetWindowDC(IntPtr.Zero);
            //blit to the graphics object g
            BitBlt(hdcDest, 0, 0, b.Width, b.Height, 
                hdcSrc, 0, 0, SRCCOPY); 
        }
        finally
        {
            //Release any native src hdcs
            if (hdcSrc != IntPtr.Zero)
            {
                ReleaseDC(hdcSrc);
                DeleteDC(hdcSrc);
            }

            //Release the graphics hdc
            g.ReleaseHdc(hdcDest);
        }
    }

    return b;
}

[DllImportAttribute("coredll.dll")]
public static extern IntPtr GetWindowDC(IntPtr hWnd);

[DllImport("coredll")]
public static extern int DeleteDC(IntPtr hdc);

[DllImportAttribute("coredll.dll")]
internal static extern IntPtr ReleaseDC(IntPtr hdc);

public const int SRCCOPY = 0x00CC0020;

[DllImport("coredll.dll")]
public static extern bool BitBlt(IntPtr hdcDest, 
    int nXDest, int nYDest, int nWidth, 
    int nHeight, IntPtr hdcSrc, int nXSrc, 
    int nYSrc, uint dwROP);

Windows Phone 7 using Silverlight

private void btnCaptureScreen_Click(object sender, RoutedEventArgs e)
{
    //Capture the screen and set it to the internal picture box
    WriteableBitmap bmp = new WriteableBitmap((int)this.ActualWidth, (int)this.ActualHeight);
    bmp.Render(this, null);
    bmp.Invalidate();
    this.image1.Source = bmp;
    
    //Set a new background
    ImageBrush brush = new ImageBrush();
    brush.ImageSource = new BitmapImage(new Uri(NextImage,UriKind.Relative));
    ContentGrid.Background = brush;
    
}

Essentially the apps do the same thing but it’s a lot easier to do it on one than the other.  Here are some quick highlights on the differences.

1. Silverlight for Windows Phone 7 has 7 lines of code while the .NET Compact Framework version has 28 lines of code
2. With Silverlight for Windows Phone 7 you don’t have to deal with PInvokes anymore.
3. Windows Phone 7 codebase is a lot smaller and more maintainable.
4. With silverlight for Windows Phone 7, controls automatically support transparencies which was a challenge on NETCF 3.5 (notice the white box on NETCF 3.5 screen, that’s a PictureBox control
5. Currently on Windows Phone 7 there is no way to get the images from your device in the form of a file unless you upload to a web service or something similar

Give it a try for yourself and download the sample code here.