C# - Queue

Queue is a special type of collection that stores the elements in FIFO style (First In First Out), exactly opposite of the Stack<T> collection. It contains the elements in the order they were added. C# includes generic Queue<T> and non-generic Queue collection. It is recommended to use the generic Queue<T> collection.

Queue<T> Characteristics

• Queue<T> is FIFO (First In First Out) collection.
• It comes under System.Collection.Generic namespace.
• Queue<T> 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. Cannot use collection-initializer syntax.
• Elements can be retrieved using the Dequeue() and the Peek() methods. It does not support an indexer.

The following figure illustrates the Queue collection:

Creating a Queue

You can create an object of the Queue<T> by specifying a type parameter for the type of elements it can store. The following example creates and adds elements in the Queue<T> using the Enqueue() method. A Queue collection allows null (for reference types) and duplicate values.

Queue<int> callerIds = new Queue<int>();
callerIds.Enqueue(1);
callerIds.Enqueue(2);
callerIds.Enqueue(3);
callerIds.Enqueue(4);

foreach(var id in callerIds)
    Console.Write(id); //prints 1234

Queue<T> Properties and Methods

Retrieve Elements from a Queue

The Dequeue() and the Peek() method is used to retrieve the first element in a queue collection. The Dequeue() removes and returns the first element from a queue because the queue stores elements in FIFO order. Calling the Dequeue() method on an empty queue will throw the InvalidOperation exception. So, always check that the total count of a queue is greater than zero before calling it.

Queue<string> strQ = new Queue<string>();
strQ.Enqueue("H");
strQ.Enqueue("e");
strQ.Enqueue("l");
strQ.Enqueue("l");
strQ.Enqueue("o");

Console.WriteLine("Total elements: {0}", strQ.Count); //prints 5

while (strQ.Count > 0)
    Console.WriteLine(strQ.Dequeue()); //prints Hello

Console.WriteLine("Total elements: {0}", strQ.Count); //prints 0

The Peek() method always returns the first item from a queue collection without removing it from the queue. Calling the Peek() method on an empty queue will throw a run-time exception InvalidOperationException.

Queue<string> strQ = new Queue<string>();
strQ.Enqueue("H");
strQ.Enqueue("e");
strQ.Enqueue("l");
strQ.Enqueue("l");
strQ.Enqueue("o");

Console.WriteLine("Total elements: {0}", strQ.Count); //prints 5

if(strQ.Count > 0){
    Console.WriteLine(strQ.Peek()); //prints H
    Console.WriteLine(strQ.Peek()); //prints H
}

Console.WriteLine("Total elements: {0}", strQ.Count); //prints 5

Contains()

The Contains() method checks whether an item exists in a queue or not. It returns true if the specified item exists, otherwise returns false.

Contains() Signature: bool Contains(object obj);

Queue<int> callerIds = new Queue<int>();
callerIds.Enqueue(1);
callerIds.Enqueue(2);
callerIds.Enqueue(3);
callerIds.Enqueue(4);

callerIds.Contains(2); //true
callerIds.Contains(10); //false