using System;

using System.Threading.Tasks;

​

public class Program

{

    public static void Main()

    {

        var s = new SomeClass();

        // call a method

        s.OnStart();

        // do some other word

        // tell the method we called that work is complete

        s.OnComplete();

        var tcs = new TaskCompletionSource<bool>();

        var s2 = new SomeOtherClass();

        s2.OnStart(tcs.Task);

        // do some other work

        tcs.SetResult(true);

    }

}

​

public class SomeClass

{

    public void OnStart()

    {

        Console.WriteLine("started");

    }

​

    public void OnComplete()

    {

        Console.WriteLine("finished");

    }

}

​

public class SomeOtherClass

{

    public void OnStart(Task<bool> isComplete)

    {

        Console.WriteLine("started");

        isComplete.ContinueWith(r => {

            Console.WriteLine("finished");  

        });

    }

}