取消 postAsync 时出现 ObjectDisposedException

Question

我的 Xamarin 应用程序中有以下两个类，用于将图像上传到服务器：

public class ProfileApi : IDisposable
{
    public Uri BaseUri { get; private set; }
    private HttpClientHandler _messageHandler;

    public ProfileApi (string baseUri, HttpClientHandler messageHandler)
    {
        BaseUri = new Uri (baseUri, UriKind.Absolute);
        _messageHandler = messageHandler;
    }

    protected HttpClient CreateHttpClient ()
    {
        var httpClient = new HttpClient (_messageHandler, true) {
            BaseAddress = BaseUri
        };
        return httpClient;
    }


    public async Task SetProfilePicture (Stream picture, CancellationToken cancellationToken)
    {
        using(var client = CreateHttpClient ())
        {
            using(var copy = new MemoryStream())
            {
                picture.Position = 0;
                picture.CopyTo(copy);
                copy.Position = 0;

                var content = new StreamContent (copy);
                content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue ("image/png");

                var response = await client.PostAsync ("post", content, cancellationToken);    

                Debug.WriteLine ("Finished!"); 
            }
        }
    }

    public void Dispose ()
    {
        _messageHandler.Dispose ();
        _messageHandler = null;
    }
}

public class ProfileDownloader
{
    static readonly Lazy<ProfileDownloader> lazy = new Lazy<ProfileDownloader> (() => 
        new ProfileDownloader ()
    );

    public static ProfileDownloader Instance {
        get {
            return lazy.Value;
        }
    }

    ProfileDownloader (){}

    CancellationTokenSource _uploadCancellationSource;

    public async Task UploadImage (Stream image)
    {
        if (AreProfileAndImageBeingUploaded ()) {
            CancelProfileAndImageUpload ();
            return;
        }

        _uploadCancellationSource = new CancellationTokenSource ();

        using (var ProfileApi = new ProfileApi ("http://httpbin.org", new NativeMessageHandler ())) 
        {
            var imageTask = ProfileApi.SetProfilePicture (image, _uploadCancellationSource.Token);

            try {

                await imageTask;
                _uploadCancellationSource = null;

            } catch (OperationCanceledException ex) {
                Debug.WriteLine (string.Format ("Cancelled! : {0}", ex.Message)); 

            } catch (Exception ex) {
                Debug.WriteLine (string.Format ("Exception! : {0}, {1}", ex.Message, ex.Source));
            }
        }
    }

    public bool AreProfileAndImageBeingUploaded ()
    {
        return _uploadCancellationSource != null;
    }

    public void CancelProfileAndImageUpload ()
    {
        if (_uploadCancellationSource != null) {
            _uploadCancellationSource.Cancel ();
            _uploadCancellationSource = null;
        }
    }
}

每当我按下应用 UI 中的按钮时，都会调用 UploadImage 方法。

var image = UIImage.FromFile ("Images/SpongeBob.png");
using (var stream = image.ToStream ()) {        
    ProfileDownloader.Instance.UploadImage (stream);
}

我就是这样做的：

按两次按钮，调用 UploadImage 两次。第一次开始上传，第二次取消，我得到 TaskCancelledException.
我又按了两次按钮。第一次开始上传，但第二次导致 ObjectDisposedException.

有谁知道为什么会抛出2.中描述的异常？

更新

这是异常的堆栈跟踪。它被抛到 await imageTask.

行

System.ObjectDisposedException: The object was used after being disposed.   at System.Net.WebConnection.EndWrite (System.Net.HttpWebRequest request, Boolean throwOnError, IAsyncResult result) [0x0002c] in 
//Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.1.64/src/mono/mcs/class/System/System.Net/WebConnection.cs:1071 at System.Net.WebConnectionStream.WriteAsyncCB (IAsyncResult r) [0x00013] in 
//Library/Frameworks/Xamarin.iOS.framework/Versions/8.10.1.64/src/mono/mcs/class/System/System.Net/WebConnectionStream.cs:458  --- End of stack trace from previous location where exception was thrown ---   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/mcs/class/corlib/System.Runtime.ExceptionServices/ExceptionDispatchInfo.cs:61    at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00047] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:201 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x0002e] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:170 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:142 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable+ConfiguredTaskAwaiter.GetResult () [0x00000] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:477 at System.Net.Http.HttpClientHandler+c__async0.MoveNext () 0x002f4] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/mcs/class/System.Net.Http/System.Net.Http/HttpClientHandler.cs:344  --- End of stack trace from previous location where exception was thrown ---   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/mcs/class/corlib/System.Runtime.ExceptionServices/ExceptionDispatchInfo.cs:61 at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00047] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:201 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x0002e] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:170 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:142 at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1+ConfiguredTaskAwaiter[System.Net.Http.HttpResponseMessage].GetResult () [0x00000] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:580 at System.Net.Http.HttpClient+c__async0.MoveNext () [0x000a9] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/mcs/class/System.Net.Http/System.Net.Http/HttpClient.cs:274  --- End of stack trace from previous location where exception was thrown ---   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/mcs/class/corlib/System.Runtime.ExceptionServices/ExceptionDispatchInfo.cs:61 at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00047] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:201 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x0002e] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:170 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:142 at System.Runtime.CompilerServices.TaskAwaiter`1[System.Net.Http.HttpResponseMessage].GetResult () [0x00000] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:372 at Shared.ProfileApi+c__async0.MoveNext () [0x00114] in 
/Users/mku/Documents/Private projects/Xamarin/Example/Shared/ProfileApi.cs:46  --- End of stack trace from previous location where exception was thrown ---   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/mcs/class/corlib/System.Runtime.ExceptionServices/ExceptionDispatchInfo.cs:61 at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00047] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:201 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x0002e] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:170 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x0000b] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:142 at System.Runtime.CompilerServices.TaskAwaiter.GetResult () [0x00000] in 
/Users/builder/data/lanes/1503/e6ebd18b/source/mono/external/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:124 at Shared.ProfileDownloader+c__async0.MoveNext () [0x000b1] in 
/Users/mku/Documents/Private projects/Xamarin/Example/Shared/ProfileDownloader.cs:42

Answer 1

您需要等待 UploadImage 否则流将在调用后立即处理。

试试这个：

var image = UIImage.FromFile ("Images/SpongeBob.png");
using (var stream = image.ToStream ()) {        
    awaProfileDownloader.Instance.UploadImage (stream);
}

您必须将包含该代码的方法更改为 async 方法。

Answer 2

原来这实际上是Mono中的一个错误，现在已经修复并将在4.2版本中发布。

Link to bug report in Xamarin's Bugzilla

取消 postAsync 时出现 ObjectDisposedException

ObjectDisposedException when canceling postAsync

c#

task-parallel-library

async-await

dotnet-httpclient

xamarin