<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <Button Content="Click" Click="cmd_Click" />
        <Canvas Name="canvas">
            <Button Canvas.Left="10" Canvas.Top="20" Content="sdfsdfsdf" />
            <Rectangle Canvas.Left="50" Canvas.Top="100" Width="50" Height="50">
                <Rectangle.Fill>
                    <VisualBrush>
                        <VisualBrush.Visual>
                            <TextBlock Text="AA" Background="Yellow" />
                        </VisualBrush.Visual>
                    </VisualBrush>
                </Rectangle.Fill>
            </Rectangle>
        </Canvas>        
    </StackPanel>
</Window>

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        public static byte[] GetScreenShot(UIElement source, Rect size, double scale, int quality)
        {

            double renderHeight = source.RenderSize.Height * scale;
            double renderWidth = source.RenderSize.Width * scale;
            RenderTargetBitmap renderTarget = new RenderTargetBitmap((int) size.Width, (int) size.Height, 94, 94, PixelFormats.Pbgra32);

            VisualBrush sourceBrush = new VisualBrush(source);

            sourceBrush.Stretch = Stretch.Uniform;
            sourceBrush.Viewbox = new Rect(0.1, 0.5, 1, 1);
            sourceBrush.ViewboxUnits = BrushMappingMode.RelativeToBoundingBox;

            DrawingVisual drawingVisual = new DrawingVisual();
            DrawingContext drawingContext = drawingVisual.RenderOpen();

            RectangleGeometry clipGeometry = new RectangleGeometry(size);
            using (drawingContext)
            {
                // drawingContext.PushClip(clipGeometry);
                drawingContext.DrawRectangle(sourceBrush, null, new Rect(0, 0, size.Width, size.Height));
            }
            renderTarget.Render(drawingVisual);

            JpegBitmapEncoder jpgEncoder = new JpegBitmapEncoder();
            jpgEncoder.QualityLevel = quality;
            jpgEncoder.Frames.Add(BitmapFrame.Create(renderTarget));

            byte[] imageArray;
            using (MemoryStream outputStream = new MemoryStream())
            {
                jpgEncoder.Save(outputStream);
                imageArray = outputStream.ToArray();
            }

            return imageArray;
        }

        private void cmd_Click(object sender, RoutedEventArgs e)
        {
            double x = 50;
            double y = 100;

            Rect rect = new Rect(x, y, 50, 50);
            byte[] screenshot = GetScreenShot(canvas, rect, 1.0, 90);

            FileStream fileStream = new FileStream("Capture.jpg", FileMode.Create, FileAccess.Write);
            BinaryWriter binaryWriter = new BinaryWriter(fileStream);
            binaryWriter.Write(screenshot);
            binaryWriter.Close();
        }
    }