I keep forgetting how to test membership or how to retrieve objects from the Hashtable. It's actually too simple to be well-documented :)
You just use
Dim table as New HashSet(Of String)
table("foo") = "foo"
To retrieve, just use
which would return "foo".
Usually you might want to store something more complex under the key, but this is all that's needed to add to a set and test set membership - which is what I usually want.