Understanding Generics—The System.Collections.Generic Namespace

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Understanding Generics Revisiting (Un)Boxing Problem with (Un)Boxing Strongly Typed Collections Boxing Issues System.Collections.Generic List<T> Type Creating Generic Methods Omission of Type Parameters Generic Structures (Classes) default Keyword Custom Generic Collection Constraining Using where Lack of Operator Constraints Generic Base Classes Creating Generic Interfaces Creating Generic Delegates Simulating Under .NET 1.1 Nested Delegates Summary

© 2006 Andrew Troelsen

The System.Collections.Generic Namespace

Generic types are found sprinkled throughout the .NET 2.0 base class libraries; however, the System.Collections.Generic namespace is chock full of them (as its name implies). Like its nongeneric counterpart (System.Collections), the System.Collections.Generic namespace contains numerous class and interface types that allow you to contain subitems in a variety of containers. Not surprisingly, the generic interfaces mimic the corresponding nongeneric types in the System.Collections namespace:

• ICollection<T>
• IComparer<T>
• IDictionary<K, V>
• IEnumerable<T>
• IEnumerator<T>
• IList<T>

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■Note By convention, generic types specify their placeholders using uppercase letters. Although any letter (or word) will do, typically T is used to represent types, K is used for keys, and V is used for values.

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The System.Collections.Generic namespace also defines a number of classes that implement many of these key interfaces. Table 10-1 describes the core class types of this namespace, the interfaces they implement, and any corresponding type in the System.Collections namespace.

Table 10-1. Classes of System.Collections.Generic

Generic Class	Nongeneric Counterpart in System.Collections	Meaning in Life
Collection<T>	CollectionBase	The basis for a generic collection
Comparer<T>	Comparer	Compares two generic objects for equality
Dictionary<K, V>	Hashtable	A generic collection of name/value pairs
List<T>	ArrayList	A dynamically resizable list of items
Queue<T>	Queue	A generic implementation of a first-in, first-out (FIFO) list
SortedDictionary<K, V>	SortedList	A generic implementation of a sorted set of name/value pairs
Stack<T>	Stack	A generic implementation of a last-in, first-out (LIFO) list
LinkedList<T>	N/A	A generic implementation of a doubly linked list
ReadOnlyCollection<T>	ReadOnlyCollectionBase	A generic implementation of a set of read-only items

The System.Collections.Generic namespace also defines a number of "helper" classes and structures that work in conjunction with a specific container. For example, the LinkedListNode<T> type represents a node within a generic LinkedList<T>, the KeyNotFoundException exception is raised when attempting to grab an item from a container using a nonexistent key, and so forth.

As you can see from Table 10-1, many of the generic collection classes have a nongeneric counterpart in the System.Collections namespace (some of which are identically named). Given that Chapter 7 illustrated how to work with these nongeneric types, I will not provide a detailed examination of each generic counterpart. Rather, I’ll make use of List<T> to illustrate the process of working with generics. If you require details regarding other members of the System.Collections.Generic namespace, consult the .NET Framework 2.0 documentation.