Is there any way to start task using ContinueWith task?

Question

My code:

var r = from x in new Task<int>(() => 1)         from y in new Task<int>(() => x + 1)          select y; r.ContinueWith(x => Console.WriteLine(x.Result)).Start();    

or

new Task<int>(() => 1)     .ContinueWith(x => x.Result + 1)     .ContinueWith(x => Console.WriteLine(x.Result))     .Start(); 

Exception:

Start may not be called on a continuation task.

So I need to start the first task. Is there any way to call last task Start method to run all tasks?

Answer 1

Any reason not to use Task.Factory.StartNew^{msdn, ms docs} for the first task? Yes, it's inconsistent - but it's fundamentally a different kind of task, in terms of being started explicitly rather than just as a continuation.

Answer 2

I'm not really sure what's wrong with just writing this:

var t1 = new Task<int>(() => 1) var r = from x in t1         from y in new Task<int>(() => x + 1)          select y; r.ContinueWith(x => Console.WriteLine(x.Result)); t1.Start(); 

or this:

var t = new Task<int>(() => 1) t.ContinueWith(x => x.Result + 1)  .ContinueWith(x => Console.WriteLine(x.Result)) t.Start(); 

That directly expresses what you actually want to do. (It's the initial task that you want to kick off. So what's wrong with invoking Start on that initial task?) Why are you looking for a syntax that obscures that?

EDIT: fixed first example...

EDIT 2 to add:

So I realise now that LinqToTasks expects task selectors to return running tasks. So the second from clause in your first example returns a task that nothing will ever run. So what you actually need is this:

var t1 = new Task<int>(() => 1); var r = from x in t1         from y in Task<int>.Factory.StartNew(() => x + 1)         select y; r.ContinueWith(x => Console.WriteLine(x.Result)); t1.Start(); 

Nothing else is going to call Start on the tasks produced in these from clauses. Since the relevant selectors don't actually get executed until the previous task completes, you're still in control of when to kick off the root task.

That appears to work, but it's pretty ugly. But that appears to be how LinqToTasks is designed... I think I'd stick with the regular function call syntax.