Async Wandering Part 7 — Exceptions on unobserved tasks

This is the seventh part of the Async Wandering series. For your convenience you can find other parts in the table of contents in Part 1 – Why creating Form from WinForms in unit tests breaks async?

Let’s take this code:

using System;
using System.Threading.Tasks;

namespace UnobservedException
{
    class Program
    {
        static void Main(string[] args)
        {
            Test();
        }

        public static async Task Test()
        {
            throw new Exception();
        }
    }
}

using System;

using System.Threading.Tasks;

namespace UnobservedException

{

class Program

{

static void Main(string[] args)

{

Test();

}

public static async Task Test()

{

throw new Exception();

}

It doesn’t print any exception because we don’t await the task. Can we do something about it?

Let’s use the UnobservedTaskException handler:

using System;
using System.Threading.Tasks;

namespace UnobservedException
{
    class Program
    {
        static void Main(string[] args)
        {
            TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException;
            Test();

            Console.ReadLine();

            GC.Collect();
            GC.WaitForPendingFinalizers();
        }

        static void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
        {
            Console.WriteLine("Unobserved exception!");
            Console.WriteLine(e.Exception);
        }

        public static async Task Test()
        {
            throw new Exception();
        }
    }
}

using System;

using System.Threading.Tasks;

namespace UnobservedException

{

class Program

{

static void Main(string[] args)

{

TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException;

Test();

Console.ReadLine();

GC.Collect();

GC.WaitForPendingFinalizers();

}

static void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)

{

Console.WriteLine("Unobserved exception!");

Console.WriteLine(e.Exception);

}

public static async Task Test()

{

throw new Exception();

}

You can now verify that it prints out the exception correctly after you press enter. When GC notices there is a task with an exception on it, it handles it and calls the delegate.

But! If you take a look at Task source it has no finalizer. How does it work?

If you decompile async method, you can see this:

// UnobservedException.Program.<Test>d__2
// Token: 0x06000006 RID: 6 RVA: 0x000020E0 File Offset: 0x000002E0
void IAsyncStateMachine.MoveNext()
{
	int num = this.<>1__state;
	try
	{
		throw new Exception();
	}
	catch (Exception exception)
	{
		this.<>1__state = -2;
		this.<>t__builder.SetException(exception);
	}
}

// UnobservedException.Program.<Test>d__2

// Token: 0x06000006 RID: 6 RVA: 0x000020E0 File Offset: 0x000002E0

void IAsyncStateMachine.MoveNext()

{

int num = this.<>1__state;

try

{

throw new Exception();

}

catch (Exception exception)

{

this.<>1__state = -2;

this.<>t__builder.SetException(exception);

}

So there is some SetException method. It creates a TaskExceptionHolder under the hood. That type has a finalizer and does the following:

AggregateException ex4 = new AggregateException(Environment.GetResourceString("TaskExceptionHolder_UnhandledException"), this.m_faultExceptions);
UnobservedTaskExceptionEventArgs unobservedTaskExceptionEventArgs = new UnobservedTaskExceptionEventArgs(ex4);
TaskScheduler.PublishUnobservedTaskException(this.m_task, unobservedTaskExceptionEventArgs);

AggregateException ex4 = new AggregateException(Environment.GetResourceString("TaskExceptionHolder_UnhandledException"), this.m_faultExceptions);

UnobservedTaskExceptionEventArgs unobservedTaskExceptionEventArgs = new UnobservedTaskExceptionEventArgs(ex4);

TaskScheduler.PublishUnobservedTaskException(this.m_task, unobservedTaskExceptionEventArgs);

So it calls the handler.