runtime: System.Linq.Except violates Linq lazy iteration paradigm

The second Enumerable is always completely iterated and not lazy iterated!

See the current implementation of ExceptIterator:

Set<TSource> set = new Set<TSource>(comparer);
set.UnionWith(second);

foreach (TSource element in first)
{
    if (set.Add(element))
    {
        yield return element;
    }
}

to fix the implementation it should look something like this (of my mind & untested):

if(comparer == null) comparer = EqualityComparer<TSource>.Default;
Set<TSource> set = new Set<TSource(comparer);

using(IEnumerator<TSource> e2 = second.GetEnumerator())
{
    foreach(TSource element = first)
    {
        if(set.Contains(element)) continue;

        while(e2.MoveNext() && !comparer.Equals(element, e2.Current))
        {
            set.Add(e2.Current);
        }

        if(set.Add(element)) yield return element;
    }
}

As an offtopic humble side note request: Please add Memoize()-functionality to .Net because Linq-starters don’t know the importance of this concept, that it even exists and that they actually need it.