Question

I have read few F# tutorials and I have noticed how easy is to perform asynchronous and parallel programming in F# compared to C#. Thus, I am trying to write an F# library which will be called from C# and take take a C# function (delegate) as a parameter and run it asynchronously.

I have managed to pass the function so far (I am even able to cancel) but what I miss is how to implement a callback back to C# which will be executed it as soon as the asynchronous operation is completed. (e.g. the function AsynchronousTaskCompleted?). Also I was wondering if I can post (e.g. Progress %) back to F# from then function AsynchronousTask.

Can someone please help me?

This is the code I have written so far (I am not familiar with F# so the following code may be wrong or poorly implemented).

//C# Code Implementation (How I make the calls/handling)
        //Action definition is: public delegate void Action();
        Action action = new Action(AsynchronousTask);
        Action cancelAction = new Action(AsynchronousTaskCancelled);
        myAsyncUtility.StartTask2(action, cancelAction);
        Debug.WriteLine("0. The task is in progress and current thread is not blocked");
        ......
        private void AsynchronousTask()
        {
            //Perform a time-consuming task
            Debug.WriteLine("1. Asynchronous task has started.");
            System.Threading.Thread.Sleep(7000);
            //Post progress back to F# progress window?
            System.Threading.Thread.Sleep(2000);
        }        
        private void AsynchronousTaskCompleted(IAsyncResult asyncResult)
        {           
            Debug.WriteLine("2. The Asynchronous task has been completed - Event Raised");
        }
        private void AsynchronousTaskCancelled()
        {
            Debug.WriteLine("3. The Asynchronous task has been cancelled - Event Raised");
        }

//F# Code Implementation
  member x.StartTask2(action:Action, cancelAction:Action) = 
        async {
            do! Async.FromBeginEnd(action.BeginInvoke, action.EndInvoke, cancelAction.Invoke)
            }|> Async.StartImmediate
        do printfn "This code should run before the asynchronous operation is completed"    
        let progressWindow = new TaskProgressWindow()
        progressWindow.Run() //This class(type in F#) shows a dialog with a cancel button
        //When the cancel button is pressed I call Async.CancelDefaultToken()

  member x.Cancel() =
        Async.CancelDefaultToken()

Answer 1

To get the benefits of F# asynchronous workflow, you have to actually write the asynchronous computation in F#. The code you’re trying to write will not work (i.e. it may run, but won’t be useful).

When you’re writing asynchronous computations in F#, you can make asynchronous calls using let! and do!. This allows you to use other primitive non-blocking computations. For example, you can use Async.Sleep instead of Thread.Sleep.

// This is a synchronous call that will block thread for 1 sec
async { do Thread.Sleep(1000) 
        someMoreStuff() }

// This is an asynchronous call that will not block threads - it will create 
// a timer and when the timer elapses, it will call 'someMoreStuff' 
async { do! Async.Sleep(1000)
        someMoreStuff() }

You can only use asynchronous operations inside the async block and it relies on the way how F# compiler handles do! and let!. There is no (easy) way to get real non-blocking execution for code that is written in sequential way (e.g. in C# or outside of async block in F#).

If you want to use F# to get the benefits of asynchronous workflows, then the best option is to implement the operations in F# and then expose them to C# using Async.StartAsTask (which gives you Task<T> that C# can easily use). Something like this:

let someFunction(n) = async {
  do! Async.Sleep(n)
  Console.WriteLine("working")
  do! Async.Sleep(n)
  Console.WriteLine("done") 
  return 10 }

type AsyncStuff() = 
  member x.Foo(n) = someFunction(n) |> Async.StartAsTask

// In C#, you can write:
var as = new AsyncStuff()
as.Foo(1000).ContinueWith(op =>
    // 'Value' will throw if there was an exception
    Console.WriteLine(op.Value))

If you don’t want to use F# (at least for the implementation of your async computations), then asynchronous workflows won’t help you. You can implement similar thing using Task, BackgroundWorker or other C# technologies (but you’ll loose the ability to run operations easily without blocking threads).