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I'm trying to write an extension method in .NET that will operate on a generic collection, and remove all items from the collection that match a given criteria. This was my first attempt: public static void RemoveWhere<T>(this ICollection<T> Coll, Func<T, bool> Criteria){ foreach (T obj in Coll.Where(Criteria)) Coll.Remove(obj); } However this throws an InvalidOperationException, "Collection was modified; enumeration operation may not execute". Which does make sense, so I made a second attempt with a second collection variable to hold the items that need to be removed and iterate through that instead: public static void RemoveWhere<T>(this ICollection<T> Coll, Func<T, bool> Criteria){ List<T> forRemoval = Coll.Where(Criteria).ToList(); foreach (T obj in forRemoval) Coll.Remove(obj); } This throws the same exception; I'm not sure I really understand why as 'Coll' is no longer the collection being iterated over, so why can't it be modified? If anyone has any suggestions as to how I can get this to work, or a better way to achieve the same, that'd be great. Thanks.

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