C# - Collections & Collection Interfaces

Collections

A collection is a set of related objects. Unlike arrays, a collection can grow and shrink dynamically as the number of objects added or deleted. A collection is a class, so you must declare a new collection before you can add elements to that collection.

This classes are specialized classes for data storage and retrieval. These classes provide support for stacks, queues, lists, and hash tables. Most collection classes implement the same interfaces.

Collection classes serve various purposes, such as allocating memory dynamically to elements and accessing a list of items on the basis of an index etc. These classes create collections of objects of the Object class, which is the base class for all data types in C#.

ArrayList	It represents ordered collection of an object that can be indexed individually. It is basically an alternative to an array. However unlike array you can add and remove items from a list at a specified position using an index and the array resizes itself automatically. It also allows dynamic memory allocation, add, search and sort items in the list.
Hashtable	t uses a key to access the elements in the collection. A hash table is used when you need to access elements by using key, and you can identify a useful key value. Each item in the hash table has a key/value pair. The key is used to access the items in the collection.
SortedList	It uses a key as well as an index to access the items in a list. A sorted list is a combination of an array and a hash table. It contains a list of items that can be accessed using a key or an index. If you access items using an index, it is an ArrayList, and if you access items using a key , it is a Hashtable. The collection of items is always sorted by the key value.
Stack	It represents a last-in, first out collection of object. It is used when you need a last-in, first-out access of items. When you add an item in the list, it is called pushing the item and when you remove it, it is called popping the item.
Queue	It represents a first-in, first out collection of object. It is used when you need a first-in, first-out access of items. When you add an item in the list, it is called enqueue and when you remove an item, it is called deque.
BitArray	It represents an array of the binary representation using the values 1 and 0. It is used when you need to store the bits but do not know the number of bits in advance. You can access items from the BitArray collection by using an integer index, which starts from zero.

Collection Interfaces

All of the collection types use some common interfaces. These common interfaces define the basic functionality for each collection class. The key collections interfaces are – IEnumerable, ICollection, IDictionary and IList.

                                    

IEnumerable acts as a base interface for all the collection types that is extended by ICollection. ICollection is further extended by IDictionary and IList.

Interface	Description
IEnumerable	Provides an enumerator which supports a simple iteration over a non-generic collection.
ICollection	Defines size, enumerators and synchronization methods for all nongeneric collections.
IDictionary	Represents a nongeneric collection of key/value pairs.
IList	Represents a non-generic collection of objects that can be individually accessed by index.

All collections interfaces are not implemented by all the collections. It depends on collection nature. For example, IDictionary interface would be implemented by only those collection classes which support key/value pairs, like HasTable and SortedList etc.