Question

A Hashtable is a collection which stores and retrieves values using a key. The key for each value must be unique, though the values themselves needn't be. Hashtables can be iterated through as a sequence of key/value pairs.

An ArrayList is an array-like collection which increases its size automatically as new elements are added. Elements can be accessed by index (starting from zero) as in the case of an array.

Both of these collections are 'weakly typed' in that elements (whatever their actual type) are always stored internally as type 'object'. This means that you may have to cast them to their actual type when you retrieve them and value types (ints, doubles etc) need to be boxed on the heap when they are stored and unboxed when they are retrieved.

However, 'strongly typed' generic equivalents exist, namely Dictionary and List, and should normally be preferred as they don't require either casting or boxing.