Queue(T) Constructor

Queue<T> Constructor ()


Initializes a new instance of the Queue<T> class that is empty and has the default initial capacity.

Namespace:   System.Collections.Generic
Assembly:  System (in System.dll)

public Queue()

The capacity of a Queue<T> is the number of elements that the Queue<T> can hold. As elements are added to a Queue<T>, the capacity is automatically increased as required by reallocating the internal array.

If the size of the collection can be estimated, specifying the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the Queue<T>.

The capacity can be decreased by calling TrimExcess.

This constructor is an O(1) operation.

The following code example demonstrates this constructor and several other methods of the Queue<T> generic class. The code example creates a queue of strings with default capacity and uses the Enqueue method to queue five strings. The elements of the queue are enumerated, which does not change the state of the queue. The Dequeue method is used to dequeue the first string. The Peek method is used to look at the next item in the queue, and then the Dequeue method is used to dequeue it.

The ToArray method is used to create an array and copy the queue elements to it, then the array is passed to the Queue<T> constructor that takes IEnumerable<T>, creating a copy of the queue. The elements of the copy are displayed.

An array twice the size of the queue is created, and the CopyTo method is used to copy the array elements beginning at the middle of the array. The Queue<T> constructor is used again to create a second copy of the queue containing three null elements at the beginning.

The Contains method is used to show that the string "four" is in the first copy of the queue, after which the Clear method clears the copy and the Count property shows that the queue is empty.

using System;
using System.Collections.Generic;

class Example
    public static void Main()
        Queue<string> numbers = new Queue<string>();

        // A queue can be enumerated without disturbing its contents.
        foreach( string number in numbers )

        Console.WriteLine("\nDequeuing '{0}'", numbers.Dequeue());
        Console.WriteLine("Peek at next item to dequeue: {0}", 
        Console.WriteLine("Dequeuing '{0}'", numbers.Dequeue());

        // Create a copy of the queue, using the ToArray method and the
        // constructor that accepts an IEnumerable<T>.
        Queue<string> queueCopy = new Queue<string>(numbers.ToArray());

        Console.WriteLine("\nContents of the first copy:");
        foreach( string number in queueCopy )

        // Create an array twice the size of the queue and copy the
        // elements of the queue, starting at the middle of the 
        // array. 
        string[] array2 = new string[numbers.Count * 2];
        numbers.CopyTo(array2, numbers.Count);

        // Create a second queue, using the constructor that accepts an
        // IEnumerable(Of T).
        Queue<string> queueCopy2 = new Queue<string>(array2);

        Console.WriteLine("\nContents of the second copy, with duplicates and nulls:");
        foreach( string number in queueCopy2 )

        Console.WriteLine("\nqueueCopy.Contains(\"four\") = {0}", 

        Console.WriteLine("\nqueueCopy.Count = {0}", queueCopy.Count);

/* This code example produces the following output:


Dequeuing 'one'
Peek at next item to dequeue: two
Dequeuing 'two'

Contents of the copy:

Contents of the second copy, with duplicates and nulls:


queueCopy.Contains("four") = True


queueCopy.Count = 0

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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