Queue<T>.GetEnumerator Method
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Returns an enumerator that iterates through the Queue<T>.
Namespace: System.Collections.Generic
Assembly: System (in System.dll)
Syntax
'Declaration
Public Function GetEnumerator As Queue<(Of <(<'T>)>)>..::..Enumerator
public Queue<(Of <(<'T>)>)>..::..Enumerator GetEnumerator()
Return Value
Type: System.Collections.Generic.Queue<T>.Enumerator
An Queue<T>.Enumerator for the Queue<T>.
Remarks
The foreach statement of the C# language (for each in C++, For Each in Visual Basic) hides the complexity of the enumerators. Therefore, using foreach is recommended, instead of directly manipulating the enumerator.
Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.
Initially, the enumerator is positioned before the first element in the collection. At this position, Current is undefined. Therefore, you must call MoveNext to advance the enumerator to the first element of the collection before reading the value of Current.
Current returns the same object until MoveNext is called. MoveNext sets Current to the next element.
If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, Current is undefined. You cannot set Current to the first element of the collection again; you must create a new enumerator instance instead.
An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and its behavior is undefined.
The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. To guarantee thread safety during enumeration, you can lock the collection during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.
Default implementations of collections in System.Collections.Generic are not synchronized.
This method is an O(1) operation.
Examples
The following code example demonstrates that the Queue<T> generic class is enumerable. The foreach statement (For Each in Visual Basic, for each in C++) is used to enumerate the queue.
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.
Imports System.Collections.Generic
Module Example
Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
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
outputBlock.Text &= number & vbCrLf
Next
outputBlock.Text &= String.Format(vbLf & "Dequeuing '{0}'", numbers.Dequeue()) & vbCrLf
outputBlock.Text &= String.Format("Peek at next item to dequeue: {0}", _
numbers.Peek()) & vbCrLf
outputBlock.Text &= String.Format("Dequeuing '{0}'", numbers.Dequeue()) & vbCrLf
' 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())
outputBlock.Text &= vbLf & "Contents of the first copy:" & vbCrLf
For Each number As String In queueCopy
outputBlock.Text &= number & vbCrLf
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)
outputBlock.Text &= String.Format(vbLf & _
"Contents of the second copy, with duplicates and nulls:") & vbCrLf
For Each number As String In queueCopy2
outputBlock.Text &= number & vbCrLf
Next
outputBlock.Text &= vbLf & String.Format("queueCopy.Contains(""four"") = {0}", _
queueCopy.Contains("four"))
outputBlock.Text &= vbLf & "queueCopy.Clear()" & vbCrLf
queueCopy.Clear()
outputBlock.Text &= String.Format(vbLf & "queueCopy.Count = {0}", _
queueCopy.Count) & vbCrLf
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
using System;
using System.Collections.Generic;
class Example
{
public static void Demo(System.Windows.Controls.TextBlock outputBlock)
{
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)
{
outputBlock.Text += number + "\n";
}
outputBlock.Text += String.Format("\nDequeuing '{0}'", numbers.Dequeue()) + "\n";
outputBlock.Text += String.Format("Peek at next item to dequeue: {0}",
numbers.Peek()) + "\n";
outputBlock.Text += String.Format("Dequeuing '{0}'", numbers.Dequeue()) + "\n";
// 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());
outputBlock.Text += "\nContents of the first copy:" + "\n";
foreach (string number in queueCopy)
{
outputBlock.Text += number + "\n";
}
// 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);
outputBlock.Text += String.Format("\nContents of the second copy, with duplicates and nulls:") + "\n";
foreach (string number in queueCopy2)
{
outputBlock.Text += number + "\n";
}
outputBlock.Text += String.Format("\nqueueCopy.Contains(\"four\") = {0}",
queueCopy.Contains("four")) + "\n";
outputBlock.Text += "\nqueueCopy.Clear()" + "\n";
queueCopy.Clear();
outputBlock.Text += String.Format("\nqueueCopy.Count = {0}", queueCopy.Count) + "\n";
}
}
/* 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
*/
Version Information
Silverlight
Supported in: 5, 4, 3
Silverlight for Windows Phone
Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0
XNA Framework
Supported in: Xbox 360, Windows Phone OS 7.0
Platforms
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.
See Also