Question

What does foreach use if a collection or list implements both IEnumerable<T> and IEnumerator<T>?

Answer 1

You can’t use a foreach on an IEnumerator<T> at all.

You can only foreach over an object that has a GetEnumerator method (whether or not it implements IEnumerable.

So to answer your question, it uses the GetEnumerator method from IEnumerable<T>.

It’s simple to prove. Just try to compile this:

IEnumerator<char> str = "asdf".GetEnumerator();
foreach (char c in str) { }