Queue<t> Class (System.Collections.Generic) | Microsoft Learn</t>

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Type Parameters

Examples

The following code example demonstrates several 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>();
        numbers.Enqueue("one");
        numbers.Enqueue("two");
        numbers.Enqueue("three");
        numbers.Enqueue("four");
        numbers.Enqueue("five");
        // A queue can be enumerated without disturbing its contents.
        foreach( string number in numbers )
            Console.WriteLine(number);
        Console.WriteLine("\nDequeuing '{0}'", numbers.Dequeue());
        Console.WriteLine("Peek at next item to dequeue: {0}",
            numbers.Peek());
        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 )
            Console.WriteLine(number);
        // 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(number);
        Console.WriteLine("\nqueueCopy.Contains(\"four\") = {0}",
            queueCopy.Contains("four"));
        Console.WriteLine("\nqueueCopy.Clear()");
        queueCopy.Clear();
        Console.WriteLine("\nqueueCopy.Count = {0}", queueCopy.Count);
/* This code example produces the following output:
three
Dequeuing 'one'
Peek at next item to dequeue: two
Dequeuing 'two'
Contents of the first copy:
three
Contents of the second copy, with duplicates and nulls:
three
queueCopy.Contains("four") = True
queueCopy.Clear()
queueCopy.Count = 0
open System
open System.Collections.Generic
let numbers = Queue()
numbers.Enqueue "one"
numbers.Enqueue "two"
numbers.Enqueue "three"
numbers.Enqueue "four"
numbers.Enqueue "five"
// A queue can be enumerated without disturbing its contents.
for number in numbers do
    printfn $"{number}"
printfn $"\nDequeuing '{numbers.Dequeue()}'"
printfn $"Peek at next item to dequeue: {numbers.Peek()}"
printfn $"Dequeuing '{numbers.Dequeue()}'"
// Create a copy of the queue, using the ToArray method and the
// constructor that accepts an IEnumerable<T>.
let queueCopy = numbers.ToArray() |> Queue
printfn $"\nContents of the first copy:"
for number in queueCopy do
    printfn $"{number}"
// Create an array twice the size of the queue and copy the
// elements of the queue, starting at the middle of the
// array.
let array2 = numbers.Count * 2 |> Array.zeroCreate
numbers.CopyTo(array2, numbers.Count)
// Create a second queue, using the constructor that accepts an
// IEnumerable(Of T).
let queueCopy2 = Queue array2
printfn $"\nContents of the second copy, with duplicates and nulls:"
for number in queueCopy2 do
    printfn $"{number}"
printfn $"""\nqueueCopy.Contains "four" = {queueCopy.Contains "four"}"""
printfn $"\nqueueCopy.Clear()"
queueCopy.Clear()
printfn $"queueCopy.Count = {queueCopy.Count}"
// This code example produces the following output:
//       one
//       two
//       three
//       four
//       five
//       Dequeuing 'one'
//       Peek at next item to dequeue: two
//       Dequeuing 'two'
//       Contents of the first copy:
//       three
//       four
//       five
//       Contents of the second copy, with duplicates and nulls:
//       three
//       four
//       five
//       queueCopy.Contains "four" = True
//       queueCopy.Clear()
//       queueCopy.Count = 0
Imports System.Collections.Generic
Module Example
    Sub Main
        Dim numbers As New Queue(Of String)
        numbers.Enqueue("one")
        numbers.Enqueue("two")
        numbers.Enqueue("three")
        numbers.Enqueue("four")
        numbers.Enqueue("five")
        ' A queue can be enumerated without disturbing its contents.
        For Each number As String In numbers
            Console.WriteLine(number)
        Console.WriteLine(vbLf & "Dequeuing '{0}'", numbers.Dequeue())
        Console.WriteLine("Peek at next item to dequeue: {0}", _
            numbers.Peek())    
        Console.WriteLine("Dequeuing '{0}'", numbers.Dequeue())
        ' Create a copy of the queue, using the ToArray method and the
        ' constructor that accepts an IEnumerable(Of T).
        Dim queueCopy As New Queue(Of String)(numbers.ToArray())
        Console.WriteLine(vbLf & "Contents of the first copy:")
        For Each number As String In queueCopy
            Console.WriteLine(number)
        ' Create an array twice the size of the queue, compensating
        ' for the fact that Visual Basic allocates an extra array 
        ' element. Copy the elements of the queue, starting at the
        ' middle of the array. 
        Dim array2((numbers.Count * 2) - 1) As String
        numbers.CopyTo(array2, numbers.Count)
        ' Create a second queue, using the constructor that accepts an
        ' IEnumerable(Of T).
        Dim queueCopy2 As New Queue(Of String)(array2)
        Console.WriteLine(vbLf & _
            "Contents of the second copy, with duplicates and nulls:")
        For Each number As String In queueCopy2
            Console.WriteLine(number)
        Console.WriteLine(vbLf & "queueCopy.Contains(""four"") = {0}", _
            queueCopy.Contains("four"))
        Console.WriteLine(vbLf & "queueCopy.Clear()")
        queueCopy.Clear()
        Console.WriteLine(vbLf & "queueCopy.Count = {0}", _
            queueCopy.Count)
    End Sub
End Module
' This code example produces the following output:
'three
'four
'five
'Dequeuing 'one'
'Peek at next item to dequeue: two
'Dequeuing 'two'
'Contents of the copy:
'three
'four
'five
'Contents of the second copy, with duplicates and nulls:
'three
'four
'five
'queueCopy.Contains("four") = True
'queueCopy.Clear()
'queueCopy.Count = 0
	Remarks
	This class implements a generic queue as a circular array. Objects stored in a Queue<T> are inserted at one end and removed from the other. Queues and stacks are useful when you need temporary storage for information; that is, when you might want to discard an element after retrieving its value. Use Queue<T> if you need to access the information in the same order that it is stored in the collection. Use Stack<T> if you need to access the information in reverse order. Use ConcurrentQueue<T> or ConcurrentStack<T> if you need to access the collection from multiple threads concurrently.
Three main operations can be performed on a Queue<T> and its elements:
Enqueue adds an element to the end of the Queue<T>.
Dequeue removes the oldest element from the start of the Queue<T>.
Peek peek returns the oldest element that is at the start of the Queue<T> but does not remove it from the Queue<T>.
The capacity of a Queue<T> is the number of elements 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. The capacity can be decreased by calling TrimExcess.
Queue<T> accepts null as a valid value for reference types and allows duplicate elements.
		ToImmutableDictionary<TSource,TKey,TValue>(IEnumerable<TSource>,
 Func<TSource,TKey>, Func<TSource,TValue>, IEqualityComparer<TKey>,
 IEqualityComparer<TValue>)
		Enumerates and transforms a sequence, and produces an immutable dictionary of its contents by using the specified key and value comparers.
		AggregateBy<TSource,TKey,TAccumulate>(IEnumerable<TSource>, Func<TSource,
TKey>, Func<TKey,TAccumulate>, Func<TAccumulate,TSource,TAccumulate>,
 IEqualityComparer<TKey>)
		Applies an accumulator function over a sequence, grouping results by key.
		GroupBy<TSource,TKey,TElement,TResult>(IEnumerable<TSource>, Func<TSource,
TKey>, Func<TSource,TElement>, Func<TKey,IEnumerable<TElement>,
TResult>, IEqualityComparer<TKey>)
		Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function.
		GroupJoin<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>,
 Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,IEnumerable<TInner>,
TResult>, IEqualityComparer<TKey>)
		Correlates the elements of two sequences based on key equality and groups the results. A specified IEqualityComparer<T> is used to compare keys.
		GroupJoin<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>,
 Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,IEnumerable<TInner>,
TResult>)
		Correlates the elements of two sequences based on equality of keys and groups the results. The default equality comparer is used to compare keys.
		Join<TOuter,TInner,TKey,TResult>(IEnumerable<TOuter>, IEnumerable<TInner>,
 Func<TOuter,TKey>, Func<TInner,TKey>, Func<TOuter,TInner,TResult>,
 IEqualityComparer<TKey>)
		Correlates the elements of two sequences based on matching keys. A specified IEqualityComparer<T> is used to compare keys.
	Thread Safety
	Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
A Queue<T> can support multiple readers concurrently, as long as the collection is not modified.  Even so, enumerating through a collection is intrinsically not a thread-safe procedure. For a thread-safe queue, see ConcurrentQueue<T>.