实现的方法的返回类型接口，Ienumerator

Question

the GetEnumerator return type is the interface itself ie Ienumerable, wondering what it means when an interface can be a method's return type; specially when its defined in that interface.

Ienumerator enum = arrayList.GetEnumerator();

Answer 1

If I understood you correctly. ArrayList doesn't implement IEnumerator , rather IEnumerable .

public class ArrayList : IList, ICollection, IEnumerable, ICloneable 

IEnumerable

public interface IEnumerable
{
    IEnumerator GetEnumerator();
}

Edit:

IEnumerator has one simple responsibility.

Supports a simple iteration over a non-generic collection.

ArrayList's responsibility is different

Implements the IList interface using an array whose size is dynamically increased as required.

May be this is required for maintaining of SRP.

ArrayList's implementation of GetEnumerator() probably returns implementation of IEnumerable which is aware of implementation details of ArrayList. But concrete type is hidden from the client behind the IEnumerator interface.