Rect bounds = VisualTreeHelper.GetDescendantBounds(canvas);
            double dpi = 96d;


            RenderTargetBitmap rtb = new RenderTargetBitmap((int)bounds.Width, (int)bounds.Height, dpi, dpi, System.Windows.Media.PixelFormats.Default);


            DrawingVisual dv = new DrawingVisual();
            using (DrawingContext dc = dv.RenderOpen())
            {
                VisualBrush vb = new VisualBrush(cnvs);
                dc.DrawRectangle(vb, null, new Rect(new Point(), bounds.Size));
            }

            rtb.Render(dv);

Canvas to image

C#