浏览器取消请求时出现ASP.NET Web API OperationCanceledException
当用户加载一个页面时，它会发出一个或多个Ajax请求，这些请求会触发ASP.NET Web API 2控制器。 如果用户导航到另一个页面，则在完成这些ajax请求之前，浏览器将取消这些请求。 我们的ELMAH HttpModule会为每个取消的请求记录两个错误：
错误1：
System.Threading.Tasks.TaskCanceledException: A task was canceled.
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at System.Web.Http.Controllers.ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext()
错误2：
System.OperationCanceledException: The operation was canceled.
at System.Threading.CancellationToken.ThrowIfCancellationRequested()
at System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__1b.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.WebHost.HttpControllerHandler.<CopyResponseAsync>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.WebHost.HttpControllerHandler.<ProcessRequestAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.TaskAsyncHelper.EndTask(IAsyncResult ar)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
看着stacktrace，我看到这里抛出了异常：https://github.com/ASP-NET-MVC/aspnetwebstack/blob/master/src/System.Web.Http.WebHost/HttpControllerHandler.cs# L413
我的问题是：我如何处理和忽略这些例外情况？
它似乎在用户代码之外...
笔记：
Applicaiton_Error TaskScheduler.UnobservedTaskException void ErrorLog_Filtering （https://code.google.com/p/elmah/wiki/ErrorFiltering） 这是ASP.NET Web API 2中的一个错误，不幸的是，我不认为有一种解决方法总是会成功的。 我们提出了一个错误来修复它。
最终，问题在于，在这种情况下，我们将已取消的任务返回给ASP.NET，并且ASP.NET将已取消的任务视为未处理的异常（它将应用程序事件日志中的问题记录下来）。
与此同时，您可以尝试下面的代码。 它添加了一个顶级消息处理程序，用于在取消令牌激发时删除内容。 如果响应没有内容，则不应触发该错误。 发生这种情况的可能性很小，因为客户端可能会在消息处理程序检查取消令牌后但在更高级别的Web API代码执行相同检查之前立即断开连接。 但我认为这在大多数情况下都会有帮助。
大卫
config.MessageHandlers.Add(new CancelledTaskBugWorkaroundMessageHandler());
class CancelledTaskBugWorkaroundMessageHandler : DelegatingHandler
{
protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
HttpResponseMessage response = await base.SendAsync(request, cancellationToken);
// Try to suppress response content when the cancellation token has fired; ASP.NET will log to the Application event log if there's content in this case.
if (cancellationToken.IsCancellationRequested)
{
return new HttpResponseMessage(HttpStatusCode.InternalServerError);
}
return response;
}
}
在为WebApi实现异常记录器时，建议扩展System.Web.Http.ExceptionHandling.ExceptionLogger类而不是创建ExceptionFilter。 WebApi内部不会为已取消的请求调用ExceptionLoggers的Log方法（但是，异常过滤器将获得它们）。 这是设计。
HttpConfiguration.Services.Add(typeof(IExceptionLogger), myWebApiExceptionLogger);
这是另一个解决此问题的解决方法。 只需在捕获OperationCanceledException的OWIN管道的开头添加一个自定义的OWIN中间件：
#if !DEBUG
app.Use(async (ctx, next) =>
{
try
{
await next();
}
catch (OperationCanceledException)
{
}
});
#endif
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