Collections in C#

Collections can be defined as a type of class used in C# for dynamic memory allocation for storing and fetching of the contents of the class, and can be used for performing multiple operations. In C#, the collections work in the form of ‘System.Collections.Generic classes’, ‘System.Collections. Concurrent classes’ and ‘System.Collections classes’. In terms of storage patterns, collections replicate the data structure of an Array, and the only difference is, unlike arrays, collections doesn’t need to be defined with the size required.

There are 3 ways to work with collections which are follow

• System.Collections.Generic classes
• System.Collections.Concurrent classes
• System.Collections classes

Examples of Collections in C#

Below are some examples of various type of collection in C#:-

Example #1 – ArrayList

It is a collection of System.Collections. It allows to hold the data of multiple data types and as the data is added, it expands automatically.

Code:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ArrayListExample
{
class Program
{
static void Main(string[] args)
{
var data = new ArrayList();   //arraylist collection
data.Add("Demo");             // add element
data.Add(1);
data.Add(5);
data.Add(26);
data.Add(56.4);
data.Add(32);
data.Remove(5);              // remove element
foreach (object obj in data)   // iteration
{
Console.WriteLine(obj);
Console.ReadLine();
}
}
}
}

In the above example, there is a collection of type ArrayList. There are some elements in ArrayList. Add() and Remove() are the methods which is used to add and remove the elements from collection respectively. foreach is used for the iteration and display the values.

Output:

Example# 2 – We will use list operation here

It is a collection of System.Collections.Generic namespace.

 Code:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Collections
{
class Program
{
static void Main(string[] args)
{
var value = new List<string>();       // list collection
value.Add("Cricket");                       // add element
value.Add("Football");
value.Add("Volleyball");
value.Add("Hockey");
value.Add("Basketball");
value.Add("Tennis");
value.Remove("Football");              // remove element
value.Remove("Tennis");
value.Insert(3, "Badminton");         // insert element
foreach (string st in value)
{
Console.WriteLine(st);
Console.ReadLine();
}
}
}
}

In the above example, the collection is of a list type. Add() and Remove() methods are used to add or remove the elements from the list respectively. Insert() is also used to insert the element in the list at a defined index. Foreach is used for iteration and display the values.

Output: 

Example# 3 : Sorted List

It consists of key and values in a collection.

Code:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Collections
{
class Program
{
static void Main(string[] args)
{
var value = new SortedList<string, int>();            // sortedlist collection
value.Add("java", 3);                                               // add element
value.Add("javascript", 4);
value.Add("c-sharp", 5);
value.Add("dotnet", 25);
value.Add("python", 27);
value.Add("typescript", 57);
foreach (var pair in value)
{
Console.WriteLine(pair);
Console.ReadLine();
}
}
}
}

In the above example, collection is of type sortedlist. There are multiple pairs of key and values in the list. It basically represents the sorted pair of keys and values.

Output:

Example# 4 : Linked list

It basically allows the sequential access of the elements.

Code:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Collections
{
class Program
{
static void Main(string[] args)
{
var value = new LinkedList<int>();             // linkedlist collection
value.AddLast(13);          // add element
value.AddLast(33);
value.AddLast(23);
value.AddLast(51);
value.AddLast(60);
value.AddFirst(4);
value.AddFirst(6);
LinkedListNode<int> node = value.Find(51);         // find the node
value.AddBefore(node, 40);
foreach (int num in value)
{
Console.WriteLine(num);
Console.ReadLine();
}
}
}
}

In the above example, collection is of type Linkedlist. AddLast() is used to place the element in the last postion whereas AddFirst() is used to place the element at first position of the list. Linkedlist consists of a node. Find() is used to find the value and place value before it.

Output :

Example# 5 – Dictionary

It consists of unique pair of keys and values.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Collections
{
class Program
{
static void Main(string[] args)
{
var pair = new Dictionary<string, string>();           // dictionary collection
pair.Add("in", "India");                                                // add keys and values
pair.Add("af", "Africa");
pair.Add("us", "United States");
pair.Add("ja", "Japan");
pair.Add("ch", "China");
pair.Add("ca", "Canada");
Console.WriteLine("Keys present in the dictionary:");
var key = new List<string>(pair.Keys);
foreach (string k in key)
{
Console.WriteLine("{0}", k);
}
Console.WriteLine("Values present in the dictionary:");
var value = new List<string>(pair.Values);
foreach (string val in value)
{
Console.WriteLine("{0}", val);
}
Console.ReadLine();
}
}
}

In above example, collection is of type dictionary which contains key and their values. Foreach is used for the iteration of keys and values.

Output 

Example# 6 – Stack

It is based on the Last-In-First-Out structure. The last element of the queue is the first one to be removed.

Code:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Collections
{
class Program
{
static void Main(string[] args)
{
var value = new Stack<int>();                          // stack collection
value.Push(10);                                                    // adding the element
value.Push(40);
value.Push(33);
value.Push(62);
value.Push(48);
value.Push(21);
value.Push(31);
Console.WriteLine(value.Pop());
Console.WriteLine(value.Peek());
Console.WriteLine();
foreach (int item in value)
{
Console.WriteLine(item);
Console.ReadLine();
}
}
}
}

In the above example, collection is of type stack. Push() is used to inserting the element at the top. Pop() is for removing and returning the element and Peek() is for returning the top element of the stack.

Output:

Example# 7 – Queue

It is based on First-In-First-Out structure. The first element of the queue is the first one to be removed.

Code:

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Collections
{
class Program
{static void Main(string[] args)
{
var value = new Queue<string>();                // queue collection
value.Enqueue("Item 1");                               // add element
value.Enqueue("Item 2");
value.Enqueue("Item 3");
value.Enqueue("Item 4");
value.Enqueue("Item 5");
value.Enqueue("Item 6");
value.Enqueue("Item 7");
Console.WriteLine(value.Dequeue());
Console.WriteLine(value.Peek());
Console.WriteLine();
foreach (string num in value)
{
Console.WriteLine(num);
Console.ReadLine();
}
}
}
}

In the above example; collection is of type queue. Enqueue() is for the inserting element at the end of the queue. Dequeue() is for removing the element from the beginning of the queue. Peek() is used for returning the item.

Output: 

So there are many ways we can use the collections. Collections are similar to an array.  Here we don’t need to define the size beforehand unlike array.

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