C# Queue

In C#, Queue is a non-generic collection class which is used to maintain a first-in, first-out(FIFO) collection of objects i.e. the first element to enter the Queue is the first one to be removed from it.

In other words, the Queue can be thought of as people waiting in a queue to enter a shopping complex, the first person at the front of the queue is the first one to exit the queue and enter the shopping complex, while a new person is always added at the back/end of the queue.

Note: Queue implements the ICollection, IEnumerable, and ICloneable interfaces.

Important features of a Queue

New elements are always added at the end of a Queue.
Elements are removed from the front of a Queue
Duplicate elements are allowed to be stored in a Queue.
Queue maintains a FIFO collection of objects, so you may use Queue, when you need to access the information in the same order in which it is stored.
To iterate over the elements of a Queue, we could use a foreach loop.

Commonly used constructors of Queue

Constructor	Description
Queue()	This constructor creates an empty instance of the Queue class by using the default initial capacity. The capacity of a Queue is the number of elements it can hold, as the new elements are added to a Queue, its capacity is automatically increased.
Queue(int capacity)	This constructor creates a Queue that is empty and has the specified initial capacity.
Queue(ICollection)	This constructor creates a Queue, initialized with the elements of an ICollection.

Properties of Queue:

Properties	Description
Count	This property gives total number of elements in a Queue.
IsSynchronzied	This property determines whether a Queue is thread-safe or not.
SyncRoot	Thie property gets an object that can be used to synchronize the access to a Queue.

Some commonly used methods of Queue class

Methods	Description
Object Dequeue()	This method removes and returns an object at the front/beginning of a Queue.
void Enqueue(Object ob)	This method inserts an object at end of a Queue.
Object Peek()	This method returns an object at the top of a Queue, without removing it.
bool Contains(Object o)	This method checks if a Queue contains a specific Object.
void Clear()	This method removes all the elements from a Queue.
IEnumerator GetEnumerator()	This method returns an IEnumerator to enumerate an entire Queue.
Object[ ] ToArray()	This method returns an Object array, containing all the elements of a Queue.

An example of Queue

In this program we will first create a Queue to hold objects and next, we will add and remove objects from this Queue by using its methods described above in the table. Let's see the code now -:

//C# Example of a Queue

using System;
using System.Collections;

class Queue1
{
public static void Main(String[] ar)
{
	//Creating a Queue
	Queue q = new Queue();
	

	//Calling the Enqueue() method to push elements into the Queue
	//New element is always added to the end of the Queue
	q.Enqueue(10);
	q.Enqueue(23);
	q.Enqueue(16);
	q.Enqueue(5);
	q.Enqueue(29);

	//Printing the contents of Queue
	Console.WriteLine("The contents of Queue: ");
	foreach(int element in q)
		Console.WriteLine(element);


	//Calling the Dequeue() method to remove the front element of the Queue
	Console.WriteLine("Removing the front element = "+ q.Dequeue());
	Console.WriteLine("Removing the next front element = "+ q.Dequeue());


	//Calling the Peek() method which only gets the front element of the Queue, without removing it
	Console.WriteLine("Peeking at the current front element = "+ q.Peek());


	//Printing the updated contents of Queue
	Console.WriteLine("Updated contents of Queue: ");
	foreach(int element in q)
		Console.WriteLine(element);
}
}

Output is - :

The contents of Queue:
10
23
16
5
29
Removing the front element = 10
Removing the next front element = 23
Peeking at the front element = 16
Updated contents of Queue:
16
5
29

Program Analysis

The method Enqueue() is called to add the elements to the Queue and its contents are printed using foreach loop.
The method Dequeue() is called, which returns and removes the first element from the Queue i.e. 10.
The method Dequeue) is called again, which returns and removes the next first element from the Queue i.e. 23.
The method Peek() is called, which returns(but doesn't remove) the element from the front element of the Queue i.e. 16. And, finally the updated contents of Queue are displayed.

Converting a Queue to an Object array

In the upcoming program, we will call the ToArray() method, which returns an Object array, containing all the elements of an invoked Queue.

//C# Example of a Queue

using System;
using System.Collections;

class Queue1
{
public static void Main(String[] ar)
{
	//Creating a Queue
	Queue q = new Queue();
	

	//Calling the Enqueue() method to push elements into the Queue
	//New element is always added to the end of the Queue
	q.Enqueue(10);
	q.Enqueue(23);
	q.Enqueue(16);
	q.Enqueue(5);
	q.Enqueue(29);

	//Printing the contents of Queue
	Console.WriteLine("The contents of Queue: ");
	foreach(int element in q)
		Console.WriteLine(element);

	//Calling the ToArray() method of Queue
	Object[] obArr = q.ToArray();


	//Printing the contents of Object array
	Console.WriteLine("The contents of Object array initialized from the elements of Queue: ");
	foreach(Object element in obArr)
		Console.WriteLine(element);
}
}

Output is :

The contents of Queue:
10
23
16
5
29
The contents of Object array initialized from the elements of Queue:
10
23
16
5
29

Program Analysis

The method Enqueue() is called to add the elements to a Queue and its contents are printed using a foreach loop.
The method ToArray() is called, which returns an Object array containing all the elements of the invoked Queue.
The objects within the Object array are iterated and displayed within a foreach loop.

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