Does foreach prefer IEnumerable<T> or IEnumerator<T>

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) { }

Answer 2

IEnumerable<T> has a method GetEnumerator - this will be an IEnumerator<T>, which is what is used for iteration (using the MoveNext method).