C# Queue

A Queue is a special type of collection that stores the elements in a First In First Out style, it is exactly the opposite of the Stack collection. It contains the elements in the order they were added.

C# has the generic Queue and non-generic Queue collection.

Queue Properties

The following are some of the common properties of a queue.

Property	Description
Count	It will return the total number of elements in the queue
IsSynchronized	It is used to get a value to indicate that access to the queue is synchronized (thread-safe) or not.

Queue Methods

The following are some of the common queue methods to perform operations like add, delete, etc., on elements of a queue.

Method	Description
Enqueue	It is used to add elements at the end of the queue.
Dequeue	It will remove and returns an item from the starting of a queue.
Clear	It will remove all the elements from the queue.
Clone	It will create a shallow copy of the queue.
Contains	It is used to determine whether an element exists in a queue or not.
Peek	It is used to get the first element from the queue.
TrimToSize	It is used to set the capacity of a queue to an actual number of elements in the queue.

Example

using System;
using System.Collections;

public class Program
{
	public static void Main()
	{
		Queue myqueue = new Queue();
		myqueue.Enqueue("Lionel");
		myqueue.Enqueue("Messi");
		myqueue.Enqueue(34);
		myqueue.Enqueue(10);
		myqueue.Enqueue("Barcelona");
		Console.WriteLine("******Queue Example******");
		Console.WriteLine("Number of Elements in Queue: {0}", myqueue.Count);
		Console.WriteLine("******Queue Elements******");
		// Access Queue Elements
		foreach (var item in myqueue)
		{
			Console.WriteLine(item);
		}
        Console.ReadLine();
	}
}

You will see the following when run

******Queue Example******
Number of Elements in Queue: 5
******Queue Elements******
Lionel
Messi
34
10
Barcelona

You can create an object of the Queue by specifying a type parameter for the type of elements it can store.

The following example creates and adds elements in the Queue using the Enqueue() method. A Queue collection allows null and duplicate values.

using System;
using System.Collections.Generic;

public class Program
{
	public static void Main()
	{
		Queue<int> lottery = new Queue<int>();
		lottery.Enqueue(11);
		lottery.Enqueue(24);
		lottery.Enqueue(43);
		lottery.Enqueue(34);
		lottery.Enqueue(4);
		lottery.Enqueue(29);
		
		foreach(var id in lottery)
			Console.Write(id + " ");
		
        Console.ReadLine();
	}
}

You will see the following

11 24 43 34 4 29

Dequeue() Method

the queue Dequeue() method will always remove and return the first element of the queue.

using System;
using System.Collections;

public class Program
{
	public static void Main()
	{
		Queue myqueue = new Queue();
		myqueue.Enqueue("Lionel");
		myqueue.Enqueue("Messi");
		myqueue.Enqueue(34);
		myqueue.Enqueue(10);
		myqueue.Enqueue("Barcelona");
		Console.WriteLine("******Queue Example******");
		Console.WriteLine("Number of Elements in Queue: {0}", myqueue.Count);
 		while (myqueue.Count > 0)
        {
            Console.WriteLine(myqueue.Dequeue());
        }
        Console.WriteLine("Number of Elements in Queue: {0}", myqueue.Count);
        Console.ReadLine();
	}
}

Run this and you will see this

******Queue Example******
Number of Elements in Queue: 5
Lionel
Messi
34
10
Barcelona
Number of Elements in Queue: 0

Peek() Method

The queue Peek() method will always return a first element of the queue

using System;
using System.Collections;

public class Program
{
	public static void Main()
	{
		Queue myqueue = new Queue();
		myqueue.Enqueue("Lionel");
		myqueue.Enqueue("Messi");
		myqueue.Enqueue(34);
		myqueue.Enqueue(10);
		myqueue.Enqueue("Barcelona");
		Console.WriteLine("******Queue Example******");
		Console.WriteLine("Number of Elements in Queue: {0}", myqueue.Count);
 		Console.WriteLine(myqueue.Peek());
        Console.WriteLine("Number of Elements in Queue: {0}", myqueue.Count);
        Console.ReadLine();
	}
}

Run this and you will see this

******Queue Example******
Number of Elements in Queue: 5
Lionel
Number of Elements in Queue: 5

Contains() Method

The queue Contains() method, you can check whether an element exists in a queue or not. In case if the element is found in the queue, then it will return true otherwise false.

using System;
using System.Collections;

public class Program
{
	public static void Main()
	{
		Queue myqueue = new Queue();
		myqueue.Enqueue("Lionel");
		myqueue.Enqueue("Messi");
		myqueue.Enqueue(34);
		myqueue.Enqueue(10);
		myqueue.Enqueue("Barcelona");
		Console.WriteLine("******Queue Example******");
		Console.WriteLine("Number of Elements in Queue: {0}", myqueue.Count);
        Console.WriteLine("Contains Element 4: {0}", myqueue.Contains(4));
        Console.WriteLine("Contains Element 34: {0}", myqueue.Contains(34));
        Console.WriteLine("Contains Key 'Lionel': {0}", myqueue.Contains("Lionel"));
		Console.WriteLine("Contains Key 'Billy': {0}", myqueue.Contains("Billy"));
        Console.WriteLine("Number of Elements in Queue: {0}", myqueue.Count);
        Console.ReadLine();
	}
}

You will see the following

******Queue Example******
Number of Elements in Queue: 5
Contains Element 4: False
Contains Element 34: True
Contains Key 'Lionel': True
Contains Key 'Billy': False
Number of Elements in Queue: 5

Notes

Queue is a First In First Out collection.
You need to add a reference to the System.Collection.Generic namespace.
Queue can contain elements of the specified type. It provides compile-time type checking and doesn’t perform boxing-unboxing because it is generic.
Elements can be added using the Enqueue() method.
Elements can be retrieved using the Dequeue() and the Peek() methods.

C#Queue