Question

I’m trying to implement Tasks in my Application.

Here’s the sample code:

There’s one simple Interface I, 3 classes are derived form it (A,B,C)
I create a list of Is, poplualte it with A, B, C instances, and then create a task for each other to call method do1();

  interface I
    {
        void do1();

    }

    class A : I
    {
        public void do1()
        {
            Console.WriteLine("A");
        }
    }


    class B : I
    {
        public void do1()
        {
            Console.WriteLine("B");
        }
    }


    class C : I
    {
        public void do1()
        {
            Console.WriteLine("C");
        }
    }

    class Program
    {
        public static void Main(string[] args)
        {
            List<I> l = new List<I>();
            l.Add(new A());
            l.Add(new B());
            l.Add(new C());


            var TaskPool = new List<Task>();


            foreach (var i in l)
            {
                Task task = new Task( () => i.do1()

                    );
                TaskPool.Add(task);
            }


            foreach (var c in TaskPool)
            {
                c.Start();
            }

            Thread.Sleep(3000);
            Console.Read();
        }


    }

I’m expecting to see

A
B
C

In the output, but instead of it I get

C
C
C

I kinda found the problem in debugger: All tasks have the same delegate, but I do not know why and how to workaround this.

Answer 1

This is a very common question. It relates to how “captured variables” work; short version, you need this:

foreach (var i in l)
{
    var copy = i;
    Task task = new Task( () => copy.do1());
    TaskPool.Add(task);
}

The problem here is that i (from the foreach) is technically declared outside the scope of the loop, and thus is captured at that outer scope; you capture the same variable each time (C# captures variables, not *values). Adding copy inside the loop-scope changes this; because of the scope, copy is captured separately per iteration.