Waiting for async/await inside a task

Question

I have this construct in my main() , which creates

var tasks = new List<Task>();

var t = Task.Factory.StartNew(
    async () =>
    {
        Foo.Fim();
        await Foo.DoBar();
    });

//DoBar not completed
t.Wait();
//Foo.Fim() done, Foo.DoBar should be but isn't

However, when I .Wait for t, it won't wait for the call to DoBar() to complete. How do I get it to actually wait?

Answer 1

It's discouraged to use Task.Factory.StartNew with async-await , you should be using Task.Run instead:

var t = Task.Run(
    async () =>
    {
        Foo.Fim();
        await Foo.DoBar();
    });

The Task.Factory.StartNew api was built before theTask-based Asynchronous Pattern (TAP) and async-await . It will return Task<Task> because you are starting a task with a lambda expression which happens to be async and so returns a task. Unwrap will extract the inner task, but Task.Run will implicitly do that for you.

---

For a deeper comparison, there's always a relevant Stephen Toub article: Task.Run vs Task.Factory.StartNew

Answer 2

It seems like I get desired functionality by Unwrap() ing the task. I'm not quite sure I get the reasoning behind this, but I suppose it works.

var t = Task.Factory.StartNew(
            async () =>
                {
                        Foo.Fim();
                        await Foo.DoBar();
                }).Unwrap();

edit: I've looked for ddescription of Unwrap() : Creates a proxy Task that represents the asynchronous operation of a Task<Task<T>> I thought this was traditionally what the task did, but if I need to call unwrap I suppose that's fine.

Answer 3

I faced similar issue recently and figured out that all you need to do is have DoBar() return some value and use .Result instead of wait.

var g = Task.Run(() => func(arg));

var val = g.Result;

This will wait for func to return its output and assign it to val.