Why do I need IEnumerator.Current in a class implementing IEnumerator<T>?

Question

I have a class that implements IEnumerator<string>. See below:

public class MyClass : IEnumerator<string>
{
    public bool MoveNext()
    {
        //....
    }

    //Implement other required methods....

   //Confusion lies below:
   public string Current { get { return this.CurrentLine; } }

   //Why do I need to implement IEnumerator.Current?!  In my tests, it's not even called    during my iteration
   object IEnumerator.Current { get { throw new NotImplementedException(); } }

}   

Besides the fact that .Current property exists on both the IEnumerator<T> interface and the IEnumerator interface (which IEnumerator<T> inherits), what's the point of implementing it? As seen above it's not even called.

Answer 1

IEnumerator<T> implements IEnumerator, so at the most basic level you have to fulfill the contract.

Specifically as to why - what happens if someone does this:

((IEnumerator)yourInstance).Current

They (usually) should expect to get a loosely-typed copy of the same value/reference returned from IEnumerator<T>'s implementation. So in most cases, just return this.Current and don't worry about it :)

(FYI - returning this.Current is also good practice because it follows DRY and SRP - let the strongly-typed version of Current deal with the implementation details of what Current actually is.)

Answer 2

The reason is that IEnumerator<T> inherits IEnumerator so when you inherit from IEnumerator<T> you implicitly also inherit from IEnumerator. If you advertise an interface you should also provide an implementation for that interface even if you never intend to use it.