Execute Task.Run on main thread

Question

I am using monotouch/Xamarin for an iOS app.

The documentation for Task.Run states:

Queues the specified work to run on the ThreadPool and returns a task handle for that work.

Which essentially indicates that it could run on any thread ThreadPool.

I want to do something like:

Task.Run(async () => await PerformTask());

but have it run on the main thread. Normally I would write it using BeginInvokeOnMainThread as follows:

BeginInvokeOnMainThread(async () => await PerformTask());

But I am doing this in shared code and do not want to use iOS specific calls. Is there a way for me to tell Task.Run() to invoke the action on the main thread?

@Servy, in my current context it is safe to make that assumption. But if there is a more general answer I would like that too. — Kostub Deshmukh, Aug 04 '15 at 16:34

score 9 · Accepted Answer · answered Aug 04 '15 at 16:36

9

If you want to run PerformTask in the current thread, instead of a thread pool thread, you simply need to not call Task.Run. Just use:

PerformTask();

and you're done. If you want to only continue executing the rest of the method when that task is done, then await it:

await PerformTask();

There is no reason to call Task.Run here for you, nor is there any reason to create a lambda that awaits the method for no particular reason (you could just call it directly if you wanted to start it from a thread pool thread).

answered Aug 04 '15 at 16:36

Servy

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One of my biggest dislikes with async/await and task is that they are lacking the feature "Execute on main". I want to do this later, but i want to do it at the next step of the main event loop (no need for locks). – Dabaus Sep 30 '21 at 17:15
@Dabaus That async methods capture and use the synchronization context means that they do that *by default*. You need to go out of your way to make them *not* do that. That's the whole point of this answer, so your comment confuses me. – Servy Sep 30 '21 at 17:18

score 2 · Answer 2 · answered Oct 08 '22 at 05:21

If you want to run a task from the main thread you could use TaskSchedluer's method FromCurrentSynchronizationContext().

Task t = new Task(() =>
{
    ...
});
t.Start(TaskScheduler.FromCurrentSynchronizationContext());

This can be useful sometimes. However, if you want to use this technique to bypass thread checks (for example, modifying properties of form controls in a windows app), you should rethink your design. Use tasks for calculations or other non-UI operations and update the UI from the main thread.

score 1 · Answer 3 · answered May 03 '17 at 09:01

1

Have a look at MainQueue.cs: https://gist.github.com/gering/0aa9750d3c7d14b856d0ed2ba98374a8

It is for Xamarin Forms applications. You have to call Init() from main thread once, but then you are able to ensure execution on main thread.

answered May 03 '17 at 09:01

Gering

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Execute Task.Run on main thread

3 Answers3