WPF中BitmapImage的StreamSource为null？为什么？

Question

 BitmapImage bmp = new BitmapImage(new Uri("Images\\DefaultPhoto.jpg",UriKind.Relative));
 byte[] byteArray = null;

        try
        {
            Stream sMarket = bmp.StreamSource;//bmp的StreamSource为null，可我不是已经赋值了吗？而且能显示图像了

            if (sMarket != null && sMarket.Length > 0)
            { 
                sMarket.Position = 0;

                using (BinaryReader br = new BinaryReader(sMarket))
                {
                    byteArray = br.ReadBytes((int)sMarket.Length);
                }
            }
        }
        catch
        {
            //other exception handling 
        }