Question

I have a foreach loop that needs converting to a for or while loop. My loop looks like this:

foreach (Item item in Items)
{
    // some stuff
}

What is the equivalent for or while loop?

I think I need to use GetEnumerator to get an IEnumerator<Item>, but I don’t know how to use it properly. Items isn’t a list, otherwise I’d use indexing.

Answer 1

In the simplest case(no disposing etc.) you can use:

var enumerator = Items.GetEnumerator();// `var` to take advantage of more specific return type
while(enumerator.MoveNext())
{
  Item item = enumerator.Current;
  ...
}

For the exact rules check the C# specification 8.8.4 The foreach statement.

A foreach statement of the form

foreach (V v in x) embedded-statement

is then expanded to:

{
  E e = ((C)(x)).GetEnumerator();
  try {
     V v;
     while (e.MoveNext()) {
        v = (V)(T)e.Current;
                embedded-statement
    }
  }
  finally {
     … // Dispose e
  }

}

(Quoted from the C# Language Specification Version 4.0)

The types using here are: “a collection type C, enumerator type E and element type T“. E is the return type of GetEnumerator, and not necessarily IEnumerator<V> since foreach uses duck typing. The spec also describes a number of corner cases and how to infer these types, but those details are probably not relevant here.

In C# 5 the declaration of v will be moved into the while loop to get more intuitive closure semantics.