尝试捕获异步等待中的异常。

Question

我遇到了一个非常奇怪的情况。我的一些异步/等待代码使用RestSharp从几个rest API（托管在不同URL上的相同API）中获取一些数据，这是一个目录服务返回的API。

现在问题来了：由该目录服务返回的其中一个API是某种“私有”的，尝试建立SSL连接失败。Fiddler捕获以下响应：

HTTP/1.1 200 Connection Established
FiddlerGateway: Direct
StartTime: 19:13:11.117
Connection: close

fiddler.network.https> HTTPS handshake to foo.bar.com failed. System.IO.IOException The handshake failed due to an unexpected packet format.

我希望在这种情况下能够跳过从此API获取数据并继续执行。然而，尝试使用try / catch语句并没有起到帮助作用！会抛出NullReferenceException异常，即使是通用的try / catch也无法捕获它。

代码非常简单：

try
{
   await GetDataAsync(url);
}
catch 
{
   // never gets called
}

并且。

private async Task<List<Data>> GetDataAsync(string url)
{
   var request = new RestRequest("/foo");
   var restClient = new RestClient(url);

   var response = await restClient.ExecuteTaskAsync<List<Data>>(request); // <-- this throws 
   return response.Data;
}

我已将代码提取到一个库中，并尝试在控制台应用程序和WPF应用程序中运行，结果相同，catch块从未被执行。有什么想法吗？附上完整的异常堆栈跟踪如下：

Unhandled Exception: System.NullReferenceException: Object reference not set to
an instance of an object.
at RestSharp.RestClient.<>c__DisplayClass15`1.<ExecuteTaskAsync>b__12(IRestResponse`1 response, RestRequestAsyncHandle _)
at RestSharp.RestClient.DeserializeResponse[T](IRestRequest request, Action`2 callback, IRestResponse response, RestRequestAsyncHandle asyncHandle)
at RestSharp.RestClient.<>c__DisplayClassa`1.<ExecuteAsync>b__9(IRestResponse response, RestRequestAsyncHandle asyncHandle)
at RestSharp.RestClient.ProcessResponse(IRestRequest request, HttpResponse httpResponse, RestRequestAsyncHandle asyncHandle, Action`2 callback)
at RestSharp.RestClient.<>c__DisplayClass3.<ExecuteAsync>b__0(HttpResponse r)
at RestSharp.Http.ExecuteCallback(HttpResponse response, Action`1 callback)
at RestSharp.Http.ResponseCallback(IAsyncResult result, Action`1 callback)
at RestSharp.Http.<>c__DisplayClass3.<GetStyleMethodInternalAsync>b__1(IAsyncResult result)
at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
at System.Net.ContextAwareResult.CompleteCallback(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Net.ContextAwareResult.Complete(IntPtr userToken)
at System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
at System.Net.HttpWebRequest.SetResponse(Exception E)
at System.Net.HttpWebRequest.CheckWriteSideResponseProcessing()
at System.Net.ConnectStream.ProcessWriteCallDone(ConnectionReturnResult returnResult)
at System.Net.HttpWebRequest.WriteCallDone(ConnectStream stream, ConnectionReturnResult returnResult)
at System.Net.ConnectStream.CallDone(ConnectionReturnResult returnResult)
at System.Net.ConnectStream.IOError(Exception exception, Boolean willThrow)
at System.Net.ConnectStream.HandleWriteHeadersException(Exception e, WebExceptionStatus error)
at System.Net.ConnectStream.WriteHeadersCallback(IAsyncResult ar)
at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
at System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
at System.Net.TlsStream.ResumeIOWorker(Object result)
at System.Net.TlsStream.WakeupPendingIO(IAsyncResult ar)
at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
at System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
at System.Net.Security.SslState.FinishHandshake(Exception e, AsyncProtocolRequest asyncRequest)
at System.Net.Security.SslState.PartialFrameCallback(AsyncProtocolRequest asyncRequest)
at System.Net.AsyncProtocolRequest.CompleteRequest(Int32 result)
at System.Net.FixedSizeReader.CheckCompletionBeforeNextRead(Int32 bytes)
at System.Net.FixedSizeReader.ReadCallback(IAsyncResult transportResult)
at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
at System.Net.ContextAwareResult.CompleteCallback(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Net.ContextAwareResult.Complete(IntPtr userToken)
at System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
at System.Net.Sockets.BaseOverlappedAsyncResult.CompletionPortCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)

- adrian h.

---

在 return response.Data; 处设置一个断点，查看 response 的内容。 - L.B

我无法到达那一行，它在ExecuteTaskAsync上抛出异常。我编辑了问题以显示抛出异常的行。 - adrian h.

你能添加异常的堆栈跟踪吗？ - i3arnon

完成，已编辑原帖 - adrian h.

如果这个异常没有被那个try-catch捕获，它在哪里被捕获了？ - Paulo Morgado

Answer 1

这似乎是RestSharp中的一个错误。我不确定你能否在不修改RestSharp源代码的情况下解决它。

- svick

---

你说得对。我也发现了 https://github.com/restsharp/RestSharp/issues/447，这基本上是相同的问题。然而，当我从GitHub获取最新的RestSharp代码以了解为什么异常会导致AppDomain崩溃时，我发现它现在已经修复了！我逐步检查了反序列化代码，现在它正确地检查是否有异常，并不再尝试反序列化响应体。 - adrian h.

我现在检出了一个较旧的RestSharp提交版本，其中仍存在错误，以尝试弄清楚为什么异常会导致应用程序崩溃。不是很确定，但据我所知，似乎RestSharp中的异步方法正在包装WebRequest上的非异步方法（例如BeginWebRequest）。我猜测，在这种情况下抛出的异常不受编译器重写异步/等待代码的支持？（并且要明确的是，当前版本的RestSharp4针对.net4进行编译，仍然不使用GetResponseAsync()，但由于实际错误已经修复，因此不再抛出NRE） - adrian h.