Why can't I modify the loop variable in a foreach?

Question

Why is a foreach loop a read only loop? What reasons are there for this?

Answer 1

I would assume it's how the iterator travels through the list.

Say you have a sorted list:

Alaska
Nebraska
Ohio

In the middle of

foreach(var s in States)
{
}

You do a States.Add("Missouri")

How do you handle that? Do you then jump to Missouri even if you're already past that index.