C# object references and Action types [duplicate]

Question

I have a quick question hopefully about Action types and Lambdas in C#. Here's come code:

    static void Main(string[] args)
    {
        List<Action> actions = new List<Action>();

        for (int I = 0; I < 10; I++)
            actions.Add(new Action(() => Print(I.ToString())));

        foreach (Action a in actions)
        {
            a.Invoke();
        }
        actions.Clear();

        int X;
        for (X = 0; X < 10; X++)
        {
            int V = X;
            actions.Add(new Action(() => Print(V.ToString())));
        }

        foreach (Action a in actions)
        {
            a.Invoke();
        }
        Console.ReadLine();
    }


    public static void Print(string s)
    {
        Console.WriteLine(s);
    }

If you run this code you will see that it outputs 10, ten times in a row, then outputs the numbers 0-9 the second time around. It clearly has something to do with the way I use X vs I, and how I give my action a new variable V each time in the second loop... Possibly that each new V is a new address in memory, but I'm struggling to understand why I.ToString() doesn't do the same thing in the first loop... Why doesn't I.ToString() used in the first Action work in the same way as the second example?

Answer 1

The for loop is effectively expanded out to this by the compiler:

{
  int I;
  for (I = 0; I < 10; I++)
  {
    actions.Add(new Action(() => Print(I.ToString())));
  }
}

This means that all the lambda instances capture the same instance of I, which will be 10 when the loop exits.

In your second example you copy the value into a variable that is scoped to the body of the for statement, and the lambda captures this local. There will be a unique local for each repetition of the loop.

It's important to realize that you don't capture the value of the variable, rather you capture the variable itself. That's why the first example doesn't work, but the second one does.