C# Collections in four minutes - Collections & Collection Interfaces

This article aims to give you an overview of collections that are available in C#.

What is a Collection?

A collection normally means a type whose purpose is to manage a group of other objects in memory and to provide access to those objects. Collection classes are specialized classes for data storage and retrieval. Collections typically include methods to add, remove, count and for, foreach for iterate through objects.
ref - https://docs.microsoft.com/en-us/dotnet/api/system.collections?view=netcore-3.1

Various Collections of C#

There are two types of collections Arrays and Advanced Collections. Advanced Collections can be divided into two. Generic & Non-Generic collections.

➤ Non-Generic Collections

The data type of the collection is not pre-defined.
These collections are the various commonly used classes of the System.Collection namespace.

• ArrayList
• SortedList
• Stack
• Queue
• Hashtable
• BitArray

I hope not to explain these data types further because these are more familiar if you are doing any programming language.

➤ Generic Collections

The data type of the collection is pre-defined.
These collections are the various commonly used classes of the System.Collection.Generic namespace.

Note:

• These collections(below) can be Generic or Non-Generic.
• T is the type of the object.
• TKey & TValue are Key and Value pairs for objects.
• Explore the methods and properties of each collection to complete your knowledge.

➧ BitArray

• A bit array is a collection of bits. 
• Bit arrays compactly store bits. 
• Most commonly, they are used to represent a simple group of boolean flags or an ordered sequence of boolean values.

➧ List<T>

➧ Dictionary<TKey, TValue>

• A dictionary is a collection of unique key/value pairs where a key is used to access the corresponding value. 
• Dictionaries are used in database indexing, cache implementations, and so on.

➧ SortedList<TKey, TValue>

➧ Stack<T>

• A stack is a Last In, First Out (LIFO) collection of elements where the last element that goes into the stack will be the first element that comes out.

➧ Queue<T>

• A queue is a First In, First Out (FIFO) collection of elements where the first element that goes into a queue is also the first element that comes out.

➧ Hashset<T>

• A hash set is a set of unique values where duplicates are not allowed.

⇢➤Further Reading

➤ Collection Interfaces

• An interface is a completely abstract class, which contains only abstract members.
• It is declared using the interface keyword.
• The interface simply describes what a class should do. The class implementing the interface must define how to accomplish the behaviors(inverse of inheritance).

These Collection Interfaces helps to transfer data from a thread to another thread without any problem. 

Note:
Normal collections are not compatible to do such work. 
Ex:- When we build a multi-thread application the UI is running in thread A. So we assign another work like sorting a list to thread B. Then the sorted list should be displayed in the UI thread(A). So a List is unable to pass between thread perfectly because objects of the list are given from memory address. But when we use IList or any other IEnumarerable Collection Interface, it creates copy a copy and pass it to the UI thread without any problem.


Some of the Collection Interfaces are in the following section.
➧ IEnumerable

• It provides an enumerator which supports a simple iteration over a non-generic collection.
• https://docs.microsoft.com/en-us/dotnet/api/system.collections.ienumerable?view=netcore-3.1

➧ ICollection

• Defines size, enumerators, and synchronization methods for all nongeneric collections.
• https://docs.microsoft.com/en-us/dotnet/api/system.collections.icollection?view=netcore-3.1

➧ IList

• Represents a non-generic collection of objects that can be individually accessed by index.
• https://docs.microsoft.com/en-us/dotnet/api/system.collections.ilist?view=netcore-3.1

➧ IDictionary

• Represents a nongeneric collection of key/value pairs.
• https://docs.microsoft.com/en-us/dotnet/api/system.collections.idictionary?view=netcore-3.1

The following diagram is for the relation between the interfaces.

ref - https://www.dotnettricks.com/learn/csharp/understanding-collections-and-collections-interfaces

Different interfaces provide different functionalities. The thing is developers should know about what to use and when to use them to achieve different functionalities.

ref - https://www.c-sharpcorner.com/UploadFile/pranayamr/collection-interface-in-net/

Summary

I hope you will enjoy the generic, non-generic collections and collections interfaces in C#. 

Your valuable feedback, question, or comments about this article are always welcome.