Question

I am implementing ICollection< T > and there are problems doing enumerator implementation.

I understand that IEnumerator< T > has to implement IEnumerator for back-compatibility (.NET 1.0)

But If i am implementing IEnumerator< T >, then there are 2 Current Properties.

I have 2 questions:

1. What should be their relation ? Is following code correct ?

   T IEnumerator<T>.Current 
   { 
       get 
       {
           if (_cursor < 0 || _cursor >= _array.Length)
               throw new InvalidOperationException("Iterator position invalid");
           else 
               return _array[_cursor];
       } 
   }
   
   
   object IEnumerator.Current
   {
       get
       {
           return IEnumerator<T>.Current;
       }
   }
   

I get this error: An object reference is required for the non-static field, method, or property ‘System.Collections.Generic.IEnumerator.Current.get’

(2). Why IEnumerator< T > has to implement IDisposable. Dispose is for unmanaged resources, but in what scenario in-general would Enumerator use unmanaged resources ?

Answer 1

There are several cases where it’s useful that IEnumerator<T> inherits from IDisposable.

For example take File.ReadLines() which needs to keep open a FileStream, which is an unmanaged resource.

Or if you think about an iterator using the yield syntax you need IDisposable to make finally clauses work correctly:

IEnumerator<int> MyIt()
{
  try
  {
    yield return 1;
  }
  finally
  {
    //Do Something
  }
}

Typically you’d implement Current like this:

T Current//Implicit interface implementation
{
  get
  {
    return something;
  }
}

object IEnumerator.Current{get{return Current;}}

Your original code doesn’t work since you try to get a static property on the interface itself. You probably wanted to to ((IEnumerator<T>)this).Current. But if you implement T Current{...} implicitly you don’t need that cast at all.