A collection that allows its elements to be garbage-collected (unless there are other references to the elements). Elements will disappear from the collection when they are garbage-collected. The WeakCollection is not thread-safe, not even for read-only access! No methods may be called on the WeakCollection while it is enumerated, not even a Contains or creating a second enumerator. The WeakCollection does not preserve any order among its contents; the ordering may be different each time the collection is enumerated. Since items may disappear at any time when they are garbage collected, this class cannot provide a useful implementation for Count and thus cannot implement the ICollection interface.