How to conditionally remove items from a .NET collection

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.