Async Wandering Part 6 — Exceptions logging

This is the sixth 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 the following code:

using System;
using System.Threading.Tasks;

namespace AggregateException
{
    class Program
    {
        static void Main(string[] args)
        {
            CreateAndAwait().Wait();
        }

        static async Task CreateAndAwait()
        {
            await CreateTask();
        }

        static Task CreateTask()
        {
            return Task.Factory.StartNew(() =>
            {
                Task.Factory.StartNew(() => { throw new Exception("FIRST TASK EXCEPTION!!!"); }, 
                    TaskCreationOptions.AttachedToParent);
                Task.Factory.StartNew(() => { throw new Exception("SECOND TASK EXCEPTION!!!"); }, 
                    TaskCreationOptions.AttachedToParent);
                Console.WriteLine("Attached both children");
            });
        }
    }
}

using System;

using System.Threading.Tasks;

namespace AggregateException

{

class Program

{

static void Main(string[] args)

{

CreateAndAwait().Wait();

}

static async Task CreateAndAwait()

{

await CreateTask();

}

static Task CreateTask()

{

return Task.Factory.StartNew(() =>

{

Task.Factory.StartNew(() => { throw new Exception("FIRST TASK EXCEPTION!!!"); },

TaskCreationOptions.AttachedToParent);

Task.Factory.StartNew(() => { throw new Exception("SECOND TASK EXCEPTION!!!"); },

TaskCreationOptions.AttachedToParent);

Console.WriteLine("Attached both children");

});

}

We create a task and add two child tasks to it, both of them throwing exceptions. What is the output?

Attached both children

Unhandled Exception: System.AggregateException: One or more errors occurred. ---> System.AggregateException: One or more errors occurred. ---> System.Exception: SECOND TASK EXCEPTION!!!
   at AggregateException.Program.<>c.<CreateTask>b__3_2() in C:\Users\afish\Desktop\msp_windowsinternals\AggregateException\Program.cs:line 35
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   --- End of inner exception stack trace ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at AggregateException.Program.<CreateAndAwait>d__2.MoveNext() in C:\Users\afish\Desktop\msp_windowsinternals\AggregateException\Program.cs:line 26
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at AggregateException.Program.Main(String[] args) in C:\Users\afish\Desktop\msp_windowsinternals\AggregateException\Program.cs:line 10

Attached both children

Unhandled Exception: System.AggregateException: One or more errors occurred. ---> System.AggregateException: One or more errors occurred. ---> System.Exception: SECOND TASK EXCEPTION!!!

at AggregateException.Program.<>c.<CreateTask>b__3_2() in C:\Users\afish\Desktop\msp_windowsinternals\AggregateException\Program.cs:line 35

at System.Threading.Tasks.Task.InnerInvoke()

at System.Threading.Tasks.Task.Execute()

--- End of inner exception stack trace ---

at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

at System.Runtime.CompilerServices.TaskAwaiter.GetResult()

at AggregateException.Program.<CreateAndAwait>d__2.MoveNext() in C:\Users\afish\Desktop\msp_windowsinternals\AggregateException\Program.cs:line 26

--- End of inner exception stack trace ---

at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)

at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)

at System.Threading.Tasks.Task.Wait()

at AggregateException.Program.Main(String[] args) in C:\Users\afish\Desktop\msp_windowsinternals\AggregateException\Program.cs:line 10

So we lost one exception. This is a case when introducing async changes semantics and doesn’t integrate with the platform nicely. How to fix that?

With this:

static Task CreateAndWait()
{
    Task action = CreateTask();
    Task faulted = action.ContinueWith(p => Console.WriteLine(p.Exception), TaskContinuationOptions.OnlyOnFaulted);
    Task succeeded = action.ContinueWith(r => { }, TaskContinuationOptions.OnlyOnRanToCompletion);
    return Task.WhenAll(faulted, succeeded);
}

static Task CreateAndWait()

{

Task action = CreateTask();

Task faulted = action.ContinueWith(p => Console.WriteLine(p.Exception), TaskContinuationOptions.OnlyOnFaulted);

Task succeeded = action.ContinueWith(r => { }, TaskContinuationOptions.OnlyOnRanToCompletion);

return Task.WhenAll(faulted, succeeded);

}

This correctly prints out both exceptions, however, we need to use normal TPL methods.