From a .NET perspective, a collection could be defined as an object that implements one or more of the System.Collections.ICollection, System.Collections.IDictionary, and System.Collections.IList interfaces. This definition leads to my classification of the "built-in" collections found in the System.Collections namespace into three broad categories:

•Ordered collections: Collections that implement only the ICollection interface are usually distinguished by the fact that insertion order controls the order in which objects may be retrieved from the collection. The System.Collections.Stack and System.Collections.Queue classes are both examples of ICollection collections.

•Indexed collections: IList-implementing collections are distinguished by the fact that their contents can be retrieved via a zero-based numeric index, like an array. The System.Collections.ArrayList object is one example of an indexed collection.

•Keyed collections: Collections implementing the IDictionary interface contain items that can be retrieved by an associated key value of some kind. The contents of IDictionary collections are also usually sorted in some fashion based on the key value and can be retrieved in sorted order by enumeration. The System.Collections.HashTable class implements the IDictionary interface