下载image或者其他文件

public void SaveDownLoadAsPNG(Image img, string filePath)
        {
            try
            {
                WebRequest request = WebRequest.Create(filePath);
                SaveFileDialog sfd = new SaveFileDialog();
                string extension = System.IO.Path.GetExtension(filePath);
                sfd.Filter = string.Format("*{0}| *{0}", extension);
                if (sfd.ShowDialog() == true)
                {
                    request.BeginGetResponse((responseAsyncCallBack) =>
                    {
                        this.Dispatcher.BeginInvoke(() =>
                        {
                            using (Stream openFileStream = sfd.OpenFile())
                            {
                                WebResponse response = request.EndGetResponse(responseAsyncCallBack);
                                Stream responseStream = response.GetResponseStream();
                                Byte[] bufferBytes = new Byte[responseStream.Length];
                                responseStream.Read(bufferBytes, 0, bufferBytes.Length);

                                openFileStream.Write(bufferBytes, 0, bufferBytes.Length);
                                openFileStream.Flush();
                            }
                        });

                    }, null);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error:" + ex.ToString());
            }
        }

public void SaveAsPNG(Image img)
        {
            SaveFileDialog sfd = new SaveFileDialog()
            {
                DefaultExt = "png",
                Filter = "Png files (*.png)|*.png",
                FilterIndex = 1
            };
            if (sfd.ShowDialog() == true)
            {
                var bitmap = new WriteableBitmap((BitmapImage)img.Source);
                EditableImage imageData = new EditableImage(bitmap.PixelWidth, bitmap.PixelHeight);
                try
                {
                    for (int y = 0; y < bitmap.PixelHeight; ++y)
                    {
                        for (int x = 0; x < bitmap.PixelWidth; ++x)
                        {
                            int pixel = bitmap.Pixels[bitmap.PixelWidth * y + x];
                            imageData.SetPixel(x, y,
                            (byte)((pixel >> 16) & 0xFF),
                            (byte)((pixel >> 8) & 0xFF),
                            (byte)(pixel & 0xFF), (byte)((pixel >> 24) & 0xFF)
                            );
                        }
                    }
                }
                catch (System.Security.SecurityException)
                {
                    throw new Exception(Messages.Msg_CanNotPrintImage);
                }
                // Save it to disk
                Stream pngStream = imageData.GetStream();
                byte[] binaryData = new Byte[pngStream.Length];
                pngStream.Read(binaryData, 0, (int)pngStream.Length);
                Stream stream = sfd.OpenFile();
                stream.Write(binaryData, 0, binaryData.Length);
                stream.Close();
                Tools.Alert(Messages.Msg_ShippingLabelImageSaveCompletely, MessageType.Information);
            }

        }