配置IClientMessageInspector后，WCF服务async/await调用丢失HttpContext问题咨询

我们的项目基于.NET Framework 4.5+，用WcfUtils生成了包含异步方法的WCF代理类。在web.config的system.serviceModel节点中配置了自定义的BehaviourExtensionElement，用来注册一个实现IClientMessageInspector的自定义检查器。这个检查器的AfterReceiveReply方法需要访问HttpContext.Current.Items[stringKey]。

现在遇到的问题是：尽管整个执行路径都没用到ConfigureAwait(false)，但在AfterReceiveReply方法里HttpContext.Current始终为null。日志显示，调用base.Channel.MyMethod异步方法的前后都存在上下文（因为WcfUtil生成的代理不使用await，只返回Task）。搜了大量资料，没找到关于MessageInspector存在时异步延续上下文的具体说明，搞不懂为什么它的同步机制和主调用不一样。

编辑1：添加中间检查器验证 #

我创建了一个虚拟的MessageInspector，发现所有await调用进入检查器时都没有上下文：当前认证用户变成了应用池用户，HttpContext.Current为null，执行堆栈如下：

à MyNamespace.InTheMiddleMessageInspector.AfterReceiveReply(Message& reply, Object correlationState) dans D:\Builds\sfpauw14759_1\_work\379\s\Client\AccueilClient\SX00.SXCLT00A.AccueilClient\InTheMiddleMessageInspector.cs:ligne 76
à System.ServiceModel.Dispatcher.ImmutableClientRuntime.AfterReceiveReply(ProxyRpc& rpc)
à System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc& rpc)
à System.ServiceModel.Channels.ServiceChannel.EndCall(String action, Object[] outs, IAsyncResult result)
à System.ServiceModel.Channels.ServiceChannelProxy.TaskCreator.<>c__DisplayClass7_0`1.<CreateGenericTask>b__0(IAsyncResult asyncResult)
à System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endFunction, Action`1 endAction, Task`1 promise, Boolean requiresSynchronization)
à System.Threading.Tasks.TaskFactory`1.<>c__DisplayClass44_0`3.<FromAsyncImpl>b__0(IAsyncResult iar)
à System.Runtime.AsyncResult.Complete(Boolean completedSynchronously)
à System.ServiceModel.Channels.ServiceChannel.SendAsyncResult.FinishSend(IAsyncResult result, Boolean completedSynchronously)
à System.ServiceModel.Channels.ServiceChannel.SendAsyncResult.SendCallback(IAsyncResult result)
à System.Runtime.Fx.AsyncThunk.UnhandledExceptionFrame(IAsyncResult result)
à System.Runtime.AsyncResult.Complete(Boolean completedSynchronously)
à System.ServiceModel.Channels.HttpChannelFactory`1.HttpRequestChannel.HttpChannelAsyncRequest.OnGetResponse(IAsyncResult result)
à System.Runtime.Fx.AsyncThunk.UnhandledExceptionFrame(IAsyncResult result)
à System.Net.LazyAsyncResult.Complete(IntPtr userToken)
à System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
à System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
à System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
à System.Net.ContextAwareResult.Complete(IntPtr userToken)
à System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
à System.Net.HttpWebRequest.ProcessResponse()
à System.Net.HttpWebRequest.SetResponse(CoreResponseData coreResponseData)
à System.Net.ConnectionReturnResult.SetResponses(ConnectionReturnResult returnResult)
à System.Net.Connection.ReadComplete(Int32 bytesRead, WebExceptionStatus errorStatus)
à System.Net.LazyAsyncResult.Complete(IntPtr userToken)
à System.Net.ContextAwareResult.Complete(IntPtr userToken)
à System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
à System.Net.Sockets.BaseOverlappedAsyncResult.CompletionPortCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
à System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)

编辑2：离开MessageInspector后上下文恢复正常 #

返回到主代码分支时，上下文恢复正常，此时执行堆栈如下：

à MyNamespace.MyClass.<MyMethodAsync>d__5`2.MoveNext()
à System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
à System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
à System.Runtime.CompilerServices.AsyncMethodBuilderCore.MoveNextRunner.Run()
à System.Web.Util.SynchronizationHelper.SafeWrapCallback(Action action)
à System.Threading.Tasks.Task.Execute()
à System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
à System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
à System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)
à System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)
à System.Threading.ThreadPoolWorkQueue.Dispatch()

看起来问题出在ASP.NET调用MessageInspector的方式上。

内容的提问来源于stack exchange，提问作者bkqc