异步任务引发NotImplementedException

Question

我继续为Windows编程而奋斗。:)

当调用该方法时：

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());
    }
}

从…

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

在类Offer中，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();
}

我把"NotImplementedException“提高了。我想我在异步调用中做错了什么，但是我不知道是什么。

异常详细信息：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()

进一步的信息:照片是我的AddNewOfferViewModel的一个AddNewOfferViewModel属性，用作图像控件的源。它是从机芯装置上的摄像机上捕捉到的：

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);
    }

当我将给出和例外的行替换为：

newOffer.PhotoBase64 = await Photo.ConvertToBase64();

我有个例外：

    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

为什么您的ConvertToBase64代码没有被期待？这是完全错误的，因为async void的结构。

尝试将代码更改为如下内容：

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

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

另外，ConvertToBase64方法的哪一行会抛出异常？

连续手术时会出现异常吗？

如果不是，那么用于async代码的一些类没有正确地实现GetAwaiter方法，这导致了异常。

如果是的话，您没有向我们展示Photo属性的问题。您的代码中可能有一些未实现的方法。

Answer 2

尝试在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()));
}