web api TaskScheduler.UnobservedTaskException - asp.net

Question

I have recently published an asp.net web api to a production web site. On production about once a day the an unhandled exception causes the web site process to recycle:
An unhandled exception occurred and the process was terminated.
Application ID: /LM/W3SVC/2/ROOT/oasis Process ID: 9236 Exception:
System.AggregateException Message: A Task's exception(s) were not
observed either by Waiting on the Task or accessing its Exception
property. As a result, the unobserved exception was rethrown by the
finalizer thread. StackTrace: at
System.Threading.Tasks.TaskExceptionHolder.Finalize() InnerException:
System.Web.HttpException Message: The remote host closed the
connection. The error code is 0x800704CD. StackTrace: at
System.Web.Http.WebHost.HttpControllerHandler.EndProcessRequest(IAsyncResult
result) at
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step,
Boolean& completedSynchronously)
This exception appears to not to originate in program code but in web api internals. I need to prevent such an excetion from recycling the process as this destroys the sessions of numberous users.
I understand that one way to handle this exception is to use :
TaskScheduler.UnobservedTaskException += (sender, e) =>
{
McpsEventLog.PostException(e.Exception, "webapicontroller");
e.SetObserved();
};
But for this to work, where can I put this code. somewhere in the web api controller? Or in the global.asax. Does someone know how the UnobserveredTaskException can be impletmented in an asp.net Web Api rest service?

Answer 1

You are right: Unhandled exception are unacceptable in the context of a web application. The right place to setup that handler is the Application_Start event.
You also should set up UnhandledException-event. It does not allow you to swallow the exception, though. Only log it.
Finally, see my question about this topic: How to treat unhandled thread-exceptions in ASP.NET? I did not get a usable answer but it might still help you.