如何将IBuffer对象保存到IsolatedStorage中的JPG文件中

Question

所以我有一个IBuffer对象，我想将其内容保存到手机独立存储中作为JPG。我尝试过以下操作，但它给了我错误NullReferenceException。

我怀疑流没有正确创建，但如何使其与IBuffer一起使用？

          using (IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication())
            {
                using (IsolatedStorageFileStream fileStream = store.CreateFile(filePath))
                {
                    MemoryStream stream = new MemoryStream();
                    photo.ImageBuffer.AsStream().CopyTo(stream);
                    BitmapImage bitmap = new BitmapImage();
                    WriteableBitmap wb = new WriteableBitmap(bitmap);

                    //Encode WriteableBitmap object to a JPEG stream.
                    Extensions.SaveJpeg(wb, fileStream, (int)photo.Dimensions.Width, (int)photo.Dimensions.Height, 0, 100);                       
                }
            }

错误消息

{System.NullReferenceException: Object reference not set to an instance of an object.
at MS.Internal.XcpImports.CheckHResult(UInt32 hr)
at MS.Internal.XcpImports.WriteableBitmap_CreateFromSource(WriteableBitmap wb, IntPtr ptrMemory, BitmapSource source, Boolean& bHasProtectedContent)
at System.Windows.Media.Imaging.WriteableBitmap..ctor(BitmapSource source)
at LensBlurX.Helpers.ISOHelper.<SaveJPG>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
at LensBlurX.Pages.PhotoPage.<SaveAsync>d__12.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
 at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
 at LensBlurX.Pages.PhotoPage.<save_Click>d__1e.MoveNext()}