WeakRefLookupTable is a "weak reference" version of the basic lookup table. This is similar to the regular LookupTable, and has the same methods; the only difference is that this type of table references its values "weakly." A value that is reachable only through weak references is subject to deletion by the garbage collector. A weak-reference lookup table is useful when you don't want a value's presence in the table to force the value to stay active, such as when the lookup table is merely a fast index to a set of values that must be otherwise reachable to be useful. When the garbage collector deletes one of our values, the key/value pair for the value is automatically deleted from the table.

intrinsic class WeakRefLookupTable :   LookupTable