Integer handled as reference type when passed into a delegate

Question

I was attending the TechDays 2013 in the Netherlands this week and I got an interesting quiz question presented. The question was: What is the output of the following progra

Answer 1

It is because it is a captured variable. Note that this used to also happen with foreach, but that changed in C# 5. But to re-write your code to what you actually have:

class Program
{
    delegate void Writer();

    class CaptureContext { // generated by the compiler and named something
        public int i;      // truly horrible that is illegal in C#
        public void DoStuff() {
            Console.WriteLine(i);
        }
    }
    static void Main(string[] args)
    {
        var writers = new List();
        var ctx = new CaptureContext();
        for (ctx.i = 0; ctx.i < 10; ctx.i++)
        {
            writers.Add(ctx.DoStuff);
        }

        foreach (Writer writer in writers)
        {
            writer();
        }
    }
}

As you can see: there is only one ctx thus only one ctx.i, and it is 10 by the time you foreach over writers.

Btw, if you want to make the old code work:

for (int tmp = 0; tmp < 10; tmp++)
{
    int i = tmp;
    writers.Add(delegate { Console.WriteLine(i); });
}

Basically, the capture-context is scoped at the same level as the variable; here the variable is scoped inside the loop, so this generates:

for (int tmp = 0; tmp < 10; tmp++)
{
    var ctx = new CaptureContext();
    ctx.i = tmp;
    writers.Add(ctx.DoStuff);
}

Here each DoStuff is on a different capture-context instance, so has a different and separate i.