Question

I’ve got an error in my build which says:

Error 12 Cannot implicitly convert type ‘System.Collections.Generic.IEnumerator< BaseClass>’ to ‘System.Collections.Generic.IEnumerator< IParentClass>’. An explicit conversion exists (are you missing a cast?)

Is it wrong to simply cast it away?

This is my code:

public Dictionary<Int32, BaseClass> Map { get; private set; }  public IEnumerator<BaseClass> GetEnumerator()         {             return this.Map.Values.GetEnumerator();         }  public IEnumerator<IParentClass> IEnumerable<IParentClass>.GetEnumerator()         {             return this.GetEnumerator(); // ERROR!         } 

My question is, can I just change this line:

return this.GetEnumerator(); 

to:

return (IEnumerator<IParentClass>)this.GetEnumerator(); 

(without any bad side effects)?

Accepted Answer:
I’ve changed the function to the following (after reading Jon Skeet’s post):

IEnumerator<IParentClass> IEnumerable<IParentClass>.GetEnumerator()         {             return this.Map.Values.Cast<IParentClass>().GetEnumerator();         } 

Answer 1

No you can’t, because generics aren’t covariant in C# at the moment. .NET itself has some support (for delegates and interfaces) but it’s not really used yet.

If you were returning IEnumerable<BaseClass> instead of IEnumerator<BaseClass> (and assuming .NEt 3.5) you could use Enumerable.Cast – but you’ll currently need to write your own extension method, e.g.

public static IEnumerator<TParent> Upcast<TParent, TChild>     (this IEnumerator<TChild> source)     where TChild : TParent {     while (source.MoveNext())     {         yield return source.Current;     } } 

Alternatively in your case you could use Cast earlier:

return this.Map.Values.Cast<BaseClass>().GetEnumerator();