Don’t fall for TaskCompletionSource traps

Wrapping callback hell with TaskCompletionSource

Ever wanted to turn callback style async code to awaitable form? You might use TaskCompletionSource for it.

  class Program
    {
        static void Main(string[] args)
        {
            Run();
            Console.ReadLine();
        }

        static async Task Run()
        {
            CallbackStyleAsyncMethod((result) => Console.WriteLine(result)); // callback style
            var asyncResult = await CallbackStyleAsyncMethodWrappedAsync(); // async/await style
            Console.WriteLine(asyncResult);
        }


        static Task<string> CallbackStyleAsyncMethodWrappedAsync()
        {
            var tsc = new TaskCompletionSource<string>();
            CallbackStyleAsyncMethod((res) => tsc.SetResult(res));
            return tsc.Task;
        }


        static void CallbackStyleAsyncMethod(Action<string> onFinish)
        {
            Task.Delay(5000).ContinueWith((_) => onFinish("I'm finished"));
        }
    }

There is one problem with above code that happens very often. In case of exception it won’t be catched in try/catch block. This exception won’t be even printed in console window/terminal unless it’s handled by TaskScheduler.UnobservedTaskException or DomainUnhandledException Event.

 try
            {
                var tsc = new TaskCompletionSource<string>();
                CallbackStyleAsyncMethod(
                    (res) => tsc.SetResult(new string[] { "a" }[1]) // runtime exception
                ); 
                return tsc.Task;
            }
            catch (Exception ex)
            {
                return Task.FromResult(""); // won't be catched
            }

Solution is trivial but easy to forget

  static Task<string> CallbackStyleAsyncMethodWrappedAsync()
        {
            try
            {
                var tsc = new TaskCompletionSource<string>();
                CallbackStyleAsyncMethod((res) =>
                {
                    try
                    {
                        tsc.SetResult(new string[] { "a" }[1]); // runtime exception
                    }
                    catch (Exception ex)
                    {
                        tsc.SetException(ex); // exception is handled here
                    }
                });
                return tsc.Task;
            }
            catch (Exception ex)
            {
                return Task.FromResult(""); // will be catched
            }
        }

Cheers!