9.1. Collections

Note

Some descriptions in this chapter use the term class. In object-oriented programming, a class is a template that defines the methods and variables for a type. For example, in Console.WriteLine(), Console is a class and WriteLine is a method defined in the class.

In addition to the value types such as int, float, bool, C# provides reference types that store references to their data (objects).

Note

With reference types, two variables can reference the same object; therefore, operations on one variable can affect the object referenced by the other variable. With value types, each variable has its own copy of the data, and it’s not possible for operations on one variable to affect the other.

When it comes to the creation and management of groups of items, there are two ways to handle the items: arrays and collections. Collections are specialized classes [1] that store series of values or objects. In terms of data structures, collections are similar to arrays, but they are designed to be more effective in the organization, store, and modification of data such as when adding, deleting, discovering, and inserting of data into the collections. Most collections support adding or removing elements (resizing) dynamically. Notably, Array doesn’t.

Collections can be divided into two groups:

  • Indexable collections: Collection elements are accessed using index such as the List<T> class, the most common indexable collection.

  • Key/value pair collections: Key/value pair collections, such as the Dictionary<TKey,TValue> class, enable access to collection elements through the key of each element.

Common types of collections include arrays, lists, dictionaries, sets, queues, and stacks, each with their specific use cases:

  • Arrays are useful when you have a fixed number of elements, and you need to access them by their indices.

  • Lists are flexible arrays; they are best when you have an unknown number of elements to store, and you need to perform various operations like adding or removing items.

  • Dictionaries are great when you need to associate keys with values and retrieve values quickly based on their keys.

  • Sets, specifically HashSets, are beneficial when you need to ensure that all elements are unique and need to check whether an element is in the collection quickly.

  • Queues and Stacks are used when you need special kinds of insertion and retrieval orders (FIFO for queues and LIFO for stacks). [2]

In this chapter, we will discuss two commonly used collections: lists and dictionaries.

Footnotes