Async Await and ContinueWith not working as expected

Question

I have the following code that runs on .NET Standard 2.0:

public static Task<JobResult> TryRunAsync(this IJob job,
                                          CancellationToken cancellationToken = default(CancellationToken))
{
    return job.RunAsync(cancellationToken)
        .ContinueWith(t => {
             if (t.IsFaulted)
                 return JobResult.FromException(t.Exception.InnerException);
             if (t.IsCanceled)
                 return JobResult.Cancelled;

             return t.Result;
         });
}

And we noticed it wasn't running as expected. We thought that when you awaited the call to TryRun it would always call the continuation which could handle the exception/cancellation and return a job result. We were hoping to reduce the amount of async state machines created... However, this is not the case it just blows up. Here is a smaller sample (create a new .net core 2.0 console app and paste the following:

using System;
using System.Threading.Tasks;

namespace ConsoleApp4
{
    public class Program
    {
        public static async Task Main()
        {
            // works
            await DoStuff();
            Console.ReadKey();

            // blows up
            await TryRun();
            Console.ReadKey();
        }

        public static Task DoStuff()
        {
            return Method()
                .ContinueWith(t => Throws())
                .ContinueWith(t => {
                    if (t.IsFaulted)
                        Console.WriteLine("Faulted");
                    else if (t.IsCompletedSuccessfully)
                        Console.WriteLine("Success");
                });
        }

        public static Task Method()
        {
            Console.WriteLine("Method");
            return Task.CompletedTask;
        }

        public static Task TryRun()
        {
            return Throws()
                .ContinueWith(t => {
                    if (t.IsFaulted)
                        Console.WriteLine("Faulted");
                    else if (t.IsCompletedSuccessfully)
                        Console.WriteLine("Success");
                });
        }

        public static Task Throws()
        {
            Console.WriteLine("Throws");
            throw new ApplicationException("Grr");
        }
    }
}

You may need <LangVersion>Latest</LangVersion> In your csproj.

UPDATE

We ended up going with the following code:

public static Task<JobResult> TryRunAsync(this IJob job, 
                                           CancellationToken cancellationToken = default(CancellationToken))
{
    var tcs = new TaskCompletionSource<JobResult>(null);
    try {
        var task = job.RunAsync(cancellationToken);
        task.ContinueWith((task2, state2) => {
            var tcs2 = (TaskCompletionSource<object>)state2;
            if (task2.IsCanceled) {
                tcs2.SetResult(JobResult.Cancelled);
            } else if (task2.IsFaulted) {
                tcs2.SetResult(JobResult.FromException(task2.Exception));
            } else {
                tcs2.SetResult(JobResult.Success);
            }
        }, tcs, cancellationToken);
    } catch (Exception ex) {
        tcs.SetResult(JobResult.FromException(ex));
    }

    return tcs.Task;
}

Answer 1

The method throws is actually throwing an exception when called, not returning a faulted Task. There is no Task for you to add a continuation to; it's simply going up the call stack before even reaching the ContinueWith call.