Helper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Media.Imaging;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media; 

namespace videomaker
{
    public class Helper
    {
        public static System.Drawing.Bitmap ConvertWindowToBitmap(Window window, int dpi)
        {
            var rtb = new RenderTargetBitmap(
                (int)window.Width, //width 
                (int)window.Width, //height 
                dpi, //dpi x 
                dpi, //dpi y 
                PixelFormats.Pbgra32 // pixelformat 
                );
            rtb.Render(window);
            var enc = new System.Windows.Media.Imaging.PngBitmapEncoder();
            BitmapFrame frame = System.Windows.Media.Imaging.BitmapFrame.Create(rtb);
            return null;
        }
        public static void SaveWindow(Window window, int dpi, string filename)
        {
            var rtb = new RenderTargetBitmap(
                (int)window.Width, //width 
                (int)window.Width, //height 
                dpi, //dpi x 
                dpi, //dpi y 
                PixelFormats.Pbgra32 // pixelformat 
                );
            rtb.Render(window);

            SaveRTBAsPNG(rtb, filename);
        }

        public static void SaveCanvas(Window window, Canvas canvas, int dpi, string filename)
        {
            Size size = new Size(window.Width, window.Height);
            canvas.Measure(size);
            //canvas.Arrange(new Rect(size)); 

            var rtb = new RenderTargetBitmap(
                (int)window.Width, //width 
                (int)window.Height, //height 
                dpi, //dpi x 
                dpi, //dpi y 
                PixelFormats.Pbgra32 // pixelformat 
                );
            rtb.Render(canvas);

            SaveRTBAsPNG(rtb, filename);
        }

        private static void SaveRTBAsPNG(RenderTargetBitmap bmp, string filename)
        {
            var enc = new System.Windows.Media.Imaging.PngBitmapEncoder();
            enc.Frames.Add(System.Windows.Media.Imaging.BitmapFrame.Create(bmp));

            using (var stm = System.IO.File.Create(filename))
            {
                enc.Save(stm);
            }
        } 


    }
}