控制台应用程序未被未处理的任务异常拆除

Question

I'm trying to figure out why my console app is not torn down by an unhandled task exception. All I do is create a task where I immediately throw an exception. Finally I force GC. In the first example I have a handler for the TaskScheduler.UnobservedTaskException event and I can see the exception get handled.

 static async Task ThrowsException()
 {
     Console.WriteLine("Throwing!");
     throw new Exception("Test exception");
}

static void Main(string[] args)
{
    TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException;
    ThrowsException();
    Console.WriteLine("Collecting garbage.");
    GC.Collect();
    GC.WaitForPendingFinalizers();
    Console.WriteLine("Done collecting garbage.");
    Console.ReadKey();
}

static void TaskScheduler_UnobservedTaskException(object sender,
   UnobservedTaskExceptionEventArgs e)
{
    Console.WriteLine("Unobserved task exception occured in finalizer.");
    Console.WriteLine(e.Exception.InnerException.Message);
}

Output:

Throwing!
Collecting garbage.
Unobserved task exception occured in finalizer.
Test exception
Done collecting garbage.

But if I comment out the line TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException the program still runs to completion. In this case the output is:

Throwing!
Collecting garbage.
Done collecting garbage.

Why doesn't the application crash in this case?

Answer 1

Not crashing the program from a unobserved task exception was a change that was made for .NET 4.5, see the remarks section of the MSDN for the event . If you want your program to have the pre .NET 4.5 behavior and cause it to crash you need to put in your app.config

<configuration> 
   <runtime> 
      <ThrowUnobservedTaskExceptions enabled="true"/> 
   </runtime> 
</configuration>

This will bring back the old behavior.

Answer 2

ThrowsException is an async method and therefore unless you'll wait on this method, the exception will be swallowed.
I assume that you need to call this method like this:

ThrowsException.Wait();

Or even catch the exception:

try
{
    ThrowsException().Wait();
}
catch (AggregateException e)
{
    // Do something
}

Also, reading the following will help you understand why it didn't crash your application: TaskScheduler.UnobservedTaskException Event .