Converting a IEnumerator to IEnumerator<T>
Tigraine
Daniel Hoelbling talks about programming and technology

Converting a IEnumerator to IEnumerator

October 11, 2009 by Daniel Hoelbling

When generics where introduced with .NET 2.0 there was a ton of 1.1 code lying around that was still built without generics. So the obvious answer by Microsoft was that most generic specialization classes can be cast to their non-generic counterparts to avoid problems for users.

Now, years later we have the opposite phenomenon. Few people are actually using untyped collections, so a new problem has come: What if you are looking at legacy code that has to call into new API that has no non-generic support.

Well, it’s simple: IEnumerator becomes IEnumerator and all is well. But there is no conversion from IEnumerator to IEnumerator, so you have to write your own little facades when trying to put square blocks into round holes:

public class CastEnumerator<T> : IEnumerator<T>
{
    private readonly IEnumerator enumerator;

    public CastEnumerator(IEnumerator enumerator)
    {
        this.enumerator = enumerator;
    }

    public void Dispose()
    {
    }

    public bool MoveNext()
    {
        return enumerator.MoveNext();
    }

    public void Reset()
    {
        enumerator.Reset();
    }

    public T Current
    {
        get { return (T)enumerator.Current; }
    }

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

The call then looks like this:

public IEnumerator<T> GetEnumerator()
{
    return new CastEnumerator<T>(untypedEnumerator);
}

As Julian Birch explained in the comments, if you are using .NET 3.5 it’s even simpler to get from an untyped IEnumerable to a IEnumerable (while not technically a IEnumerator, the typed one will then return a IEnumerator):

IEnumerable untypedEnumerable = ...;
IEnumerable<string> typedEnumerable = untypedEnumerable.Cast<T>();

View Comments