What is the type I need to store for this asynchronous function?

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5.00/5 (1 vote)

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Hi,

I have a class which contains a function that needs to be called asynchronously at a later time. Similar to this:

C#

public class MyClass
{
    public Task SomeFunctionAsync()
    {
        // Do a long-running thing...
    }
}

I actually have a few of these classes, and I am wanting to store references to these Task SomeFunctionAsync() functions in a List<> or similar so that I can invoke these functions later. But I am struggling with the type I need to store.

I'm sure the answer will be obvious when I see it, but I'd really appreciate someone to clear up my muddy thinking here.

Kind wishes ~ Patrick

What I have tried:

Currently I am trying to store these in a List<Task>, which compiles but I can't work out how to invoke the stored delegate.

Posted 22-Mar-16 3:26am

Patrick Skelton

Updated 22-Mar-16 3:49am

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2 solutions

Solution 1

visit Here to understand....
Asynchronous Method Invocation[^]

Posted 22-Mar-16 3:32am

Rakesh Meel

Comments

Patrick Skelton 22-Mar-16 10:36am

It is a nice article, but I don't have the System.Windows.Forms namespace available, because I am in a Xamarin.Forms application. Is there a 'more generic' alternative?

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Richard Deeming · Accepted Answer · 2016-03-22T03:49:00

Solution 2

If you store the results in a List<Task>, the methods will be invoked once, and you will be able to retrieve the results from the list.

If you want to invoke the methods multiple times on demand, store them in a List<Func<Task>>:

C#

var listOfDelegates = new List<Func<Task>>
{
    instanceOfMyClass.SomeFunctionAsync
};
...
List<Task> listOfTasks = listOfDelegates.Select(fn => fn()).ToList();
await Task.WhenAll(listOfTasks);

Posted 22-Mar-16 3:49am

Richard Deeming

Updated 22-Mar-16 3:56am

v2

Comments

Sascha Lefèvre 22-Mar-16 9:56am

+5Typo in the last line

Richard Deeming 22-Mar-16 9:57am

Fixed. :)

Patrick Skelton 22-Mar-16 10:34am

Sorry, so are you saying that the async method will be triggered as soon as I add it to the List?

Richard Deeming 22-Mar-16 10:36am

If you use List<Task>, then yes. It is possible to create Tasks that haven't started yet, but it's not recommended, and the majority of task-returning APIs don't do it.

Patrick Skelton 22-Mar-16 10:45am

Ooo... seems like I suddenly have more reading to do. I've tried to do thorough research on this. I've even bought a couple of books, which admittedly I have only just started reading. But I have not read anything to indicate why putting Tasks in a list and starting them later is a bad idea. I ~think~ I understand why, but it is also obvious there is a big hole in my knowledge. Thank you.

Richard Deeming 22-Mar-16 10:54am

The PFX Team blog[^] is worth a read.

This one[^] explains why it's usually a bad idea to return a Task that hasn't been scheduled.

Patrick Skelton 22-Mar-16 11:13am

I see. Think I need to do some restructuring of my code.