异步任务引发 NotImplementedException

Question

我继续在 Windows Phone 编程方面苦苦挣扎。 :)

调用方法时：

public static async Task<string> ConvertToBase64(this BitmapImage bitmapImage)
{
    RandomAccessStreamReference rasr = RandomAccessStreamReference.CreateFromUri(bitmapImage.UriSource);
    var streamWithContent = await rasr.OpenReadAsync();
    byte[] buffer = new byte[streamWithContent.Size];
    var result = await streamWithContent.ReadAsync(buffer.AsBuffer(), (uint)streamWithContent.Size, InputStreamOptions.None);
    using (MemoryStream ms = new MemoryStream(result.ToArray()))
    {
        return Convert.ToBase64String(ms.ToArray());
    }
}

来自

public async void SetPhotoString(BitmapImage bi)
{
    photoBase64 = await bi.ConvertToBase64();
}

在类Offer中来自按钮回调：

public async void Post()
{
    var newOffer = new Offer()
    {
        BookAuthor = Author,
        BookTitle = Title,
        CurrencyWorth = Price,
        Description = Description,
        StartedAt = DateTime.UtcNow,
        Status = OfferStatus.Added
    };
    newOffer.SetPhotoString(Photo);//this line throws an excp.
    var result = await offerService.AddOffer(newOffer);
    if (result != null)
        ClearFormula();
}

我得到了“NotImplementedException”引发。我想我对异步调用做错了，但我不知道是什么。

异常详情： message: The method or operation is not implemented.

    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at SecondHandBookshop.Shared.Helpers.BitmapImageExtensions.<ConvertToBase64>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`1.GetResult()
   at SecondHandBookshop.Shared.Models.Offer.<SetPhotoString>d__0.MoveNext()

更多信息： Photo 是我的 AddNewOfferViewModel 的 BitmapImage 属性，它充当 Image 控件的源。它是从虚拟设备上的相机捕获的：

public async void TakeAPhoto()
    {
        ImageEncodingProperties imgFormat = ImageEncodingProperties.CreateJpeg();
        StorageFile file = await ApplicationData.Current.LocalFolder.CreateFileAsync("TestPhoto.jpg",
            CreationCollisionOption.GenerateUniqueName);
        await _mediaCapture.CapturePhotoToStorageFileAsync(imgFormat, file);
        Photo = new BitmapImage(new Uri(file.Path));
        await MediaCapture.StopPreviewAsync();
        NotifyOfPropertyChange(() => Photo);
        ShowCaptureFrame = false;
        NotifyOfPropertyChange(() => ShowCaptureFrame);
    }

当我将给出和异常的行替换为：

newOffer.PhotoBase64 = await Photo.ConvertToBase64();

我得到一个例外：

    Additional information: Value does not fall within the expected range.

       at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at SecondHandBookshop.Shared.Helpers.BitmapImageExtensions.<ConvertToBase64>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`1.GetResult()
   at SecondHandBookshop.WindowsPhone.ViewModels.AddOfferViewModel.<Post>d__1.MoveNext()

Answer 1

为什么没有等待您的ConvertToBase64 代码？这是完全错误的，因为async void 构造。

试着把你的代码改成这样：

newOffer.photoBase64 = await SetPhotoString(Photo);//this line throws an excp.

public async Task<string> SetPhotoString(BitmapImage bi)
{
    return await bi.ConvertToBase64();
}

另外，ConvertToBase64 方法的哪一行会引发异常？
连续操作是否会出现异常？
如果不是，那么您用于async 代码的某些类没有正确实现GetAwaiter 方法，这会导致您出现异常。
如果是，那么问题出在Photo 属性中，您没有向我们展示。可能是您的代码中有一些未实现的方法。

Answer 2

尝试在您的 ConvertToBase64 扩展方法中返回一个任务。

using (MemoryStream ms = new MemoryStream(result.ToArray()))
{
    return Task.FromResult(Convert.ToBase64String(ms.ToArray()));
}