wpf 保存图片到任意格式jpg,png,bmp

private void ExportBtn_Click(object sender, RoutedEventArgs e)
        {
            SaveFileDialog saveFileDialog = new SaveFileDialog();
            saveFileDialog.Filter = "jpg文件|*.jpg|png文件|*.png|bmp文件|*.bmp|所有文件|*.*";
            saveFileDialog.RestoreDirectory = true;
            saveFileDialog.ShowDialog();

            string path = saveFileDialog.FileName;
            if (path == "")
            {
                return;
            }

            var image = (this.imagListBox.SelectedItem as PictureInfo).PicturePath;//图片的文件路径
            string pictureName = saveFileDialog.FileName;

            using (MemoryStream mem = new MemoryStream())
            {
                //这句很重要，不然不能正确保存图片或出错（关键就这一句）
                System.Drawing.Bitmap bmp = new System.Drawing.Bitmap(image);
                string extension = System.IO.Path.GetExtension(path);//扩展名
                                                                     //保存到磁盘文件
                if (extension == ".jpg")
                {
                    bmp.Save(@pictureName, System.Drawing.Imaging.ImageFormat.Jpeg);

                }
                else if (extension == ".png")
                {
                    bmp.Save(@pictureName, System.Drawing.Imaging.ImageFormat.Png);
                }
                else if (extension == ".bmp")
                {
                    bmp.Save(@pictureName, System.Drawing.Imaging.ImageFormat.Bmp);
                }
                else
                {
                    bmp.Save(@pictureName, System.Drawing.Imaging.ImageFormat.Tiff);
                }

                bmp.Dispose();
                MessageBox.Show("导出成功！");
            }
        }