Question

The following program

static IEnumerable<Action> Create()
{
    foreach (var i in Enumerable.Range(0, 2))
    {
        yield return () => { Console.Write(i); };
    }
}

static void Main(string[] args)
{
    foreach (var func in Create())
    {
        func();
    }

    Console.ReadLine();
}

ouputs

01

And this program

static event Action SomethingHappened;

static void Register()
{
    foreach (var i in Enumerable.Range(0, 2))
    {
        SomethingHappened += () => { Console.Write(i); };
    }
}

static void Main(string[] args)
{
    Register();
    SomethingHappened();
    Console.ReadLine();
}

outputs

11

Why is that so? How to make the program 2 output 01?

Answer 1

You’re capturing the loop variable in your lambda expression. That means when the delegate is finally invoked, it will use the latest value of the variable… which will always be 1. Try this:

foreach (var i in Enumerable.Range(0, 2))
{
    int copy = i;
    SomethingHappened += () => { Console.Write(copy); };
}

… then read Eric Lippert’s blog post about it.