CastLookup.cs

        public static CastLookup<TKey, TElement> Cast<TKey, TElement>(this ILookup<TKey, TElement> lookup)
        {
            return new CastLookup<TKey,TElement>(lookup);
        }

        public class CastLookup<TKey, TElement>
        {
            private readonly ILookup<TKey, TElement> lookup;

            public CastLookup(ILookup<TKey, TElement> lookup)
            {
                this.lookup = lookup;
            }

            public ILookup<TKey, TNewElement> To<TNewElement>() 
            {
                return new Lookup<TNewElement>(this.lookup);
            }

            private class Lookup<TNewElement> : ILookup<TKey, TNewElement> 
            {
                private readonly ILookup<TKey, TElement> innerLookup;

                private class Grouping: IGrouping<TKey, TNewElement>
                {
                    private readonly IGrouping<TKey, TElement> innerGrouping;
                    public Grouping(IGrouping<TKey, TElement> innerGrouping )
                    {
                        this.innerGrouping = innerGrouping;
                    }

                    public TKey Key { get { return innerGrouping.Key; } }
                    public IEnumerator<TNewElement> GetEnumerator() { return innerGrouping.Cast<TNewElement>().GetEnumerator(); }
                    IEnumerator IEnumerable.GetEnumerator() { return  innerGrouping.GetEnumerator(); }
                }

                public Lookup(ILookup<TKey, TElement> innerLookup)
                {
                    this.innerLookup = innerLookup;
                }

                public IEnumerator<IGrouping<TKey, TNewElement>> GetEnumerator()
                {
                    return innerLookup.Select(g => new Grouping(g)).GetEnumerator();
                }

                IEnumerator IEnumerable.GetEnumerator()
                {
                    return GetEnumerator();
                }

                public bool Contains(TKey key)
                {
                    return innerLookup.Contains(key);
                }

                public int Count
                {
                    get { return innerLookup.Count; }
                }

                public IEnumerable<TNewElement> this[TKey key]
                {
                    get { return innerLookup[key].Cast<TNewElement>(); }
                }
            }

        }