保存Image控件source 至本地出错,该怎么解决

string message = "保存成功";            try            {                BitmapSource source = ImagePanel.Source as BitmapSource;                if (source == null)                    return;                WriteableBitmap wb = new WriteableBitmap(source);                              using (MemoryStream stream = new MemoryStream(50000))                {                    wb.SaveJpeg(stream, wb.PixelWidth, wb.PixelHeight, 0, 100);                    stream.Seek(0, SeekOrigin.Begin);                    Guid guid = Guid.NewGuid();                    try                    {                        PhoneUtils.SavePicture(stream, guid.ToString() + ".jpg");                    }                    catch (Exception ex)                    {                        message = "保存失败,错误：" + ex.Message;                    }                }            }            catch (Exception ex)            {                message = ex.Message;            }