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Queue(Of T).Dequeue Method

Removes and returns the object at the beginning of the Queue(Of T).

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

'Declaration
Public Function Dequeue As T

Return Value

Type: T
The object that is removed from the beginning of the Queue(Of T).

ExceptionCondition
InvalidOperationException

The Queue(Of T) is empty.

This method is similar to the Peek method, but Peek does not modify the Queue(Of T).

If type T is a reference type, Nothing can be added to the Queue(Of T) as a value.

This method is an O(1) operation.

The following code example demonstrates several methods of the Queue(Of T) generic class, including the Dequeue method.

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(Of T) constructor that takes IEnumerable(Of 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(Of 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.

Imports System
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)
        Next

        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)
        Next 

        ' 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)
        Next

        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: 

'one 
'two 
'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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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