Queue<T>.GetEnumerator 方法

参考

定义

命名空间:: System.Collections.Generic

程序集:: System.Collections.dll

程序集:: System.dll

程序集:: netstandard.dll

Source:: Queue.cs

Source:: Queue.cs

Source:: Queue.cs

重要

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返回循环访问 Queue<T> 的枚举数。

public:
 System::Collections::Generic::Queue<T>::Enumerator GetEnumerator();

public System.Collections.Generic.Queue<T>.Enumerator GetEnumerator ();

member this.GetEnumerator : unit -> System.Collections.Generic.Queue<'T>.Enumerator

Public Function GetEnumerator () As Queue(Of T).Enumerator

Queue<T>.Enumerator

Queue<T>.Enumerator 的一个 Queue<T>。

示例

下面的代码示例演示泛 Queue<T> 型类是可枚举的。在 foreach Visual Basic for each 中，For Each在 C++) 中 (语句用于枚举队列。

该代码示例创建一个具有默认容量的字符串队列， Enqueue 并使用方法对五个字符串进行排队。枚举队列的元素，这不会更改队列的状态。方法 Dequeue 用于取消第一个字符串的排队。方法 Peek 用于查看队列中的下一项，然后使用 Dequeue 方法将其取消排队。

ToArray方法用于创建数组并将队列元素复制到其中，然后将该数组传递给Queue<T>采用 IEnumerable<T>的构造函数，创建队列的副本。将显示副本的元素。

创建两倍于队列大小的数组，并使用 CopyTo 方法从数组中间开始复制数组元素。构造 Queue<T> 函数再次用于在开头创建包含三个 null 元素的队列的第二个副本。

方法 Contains 用于显示字符串“four”位于队列的第一个副本中，之后 Clear 方法清除副本，属性 Count 显示队列为空。

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:

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
 */

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)
        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

注解

foreach Visual Basic) for each C++ For Each 中的 C# 语言 (语句隐藏枚举器的复杂性。因此，建议使用 foreach，而不是直接操作枚举数。

枚举器可用于读取集合中的数据，但不能用于修改基础集合。

最初，枚举数定位在集合中第一个元素的前面。在此位置上，未定义 Current。因此，在读取 MoveNext 的值之前，必须调用 Current 将枚举器向前移动到集合的第一个元素。

在调用 Current 之前，MoveNext 返回相同的对象。 MoveNext 将 Current 设置为下一个元素。

如果 MoveNext 传递集合的末尾，则枚举器位于集合中最后一个元素之后，并 MoveNext 返回 false。当枚举器位于此位置时，对 MoveNext 的后续调用也会返回 false。如果最后一次MoveNext调用返回，falseCurrent则为未定义。无法再次将 Current 设置为集合的第一个元素；必须改为创建新的枚举器实例。

只要集合保持不变，枚举器就仍有效。如果对集合进行了更改（例如添加、修改或删除元素），枚举器将不可恢复地失效，并且对或 IEnumerator.Reset 的下一次InvalidOperationException调用MoveNext将引发。

枚举数没有对集合的独占访问权；因此，从头到尾对一个集合进行枚举在本质上不是一个线程安全的过程。若要确保枚举过程中的线程安全性，可以在整个枚举过程中锁定集合。若要允许多个线程访问集合以进行读写操作，则必须实现自己的同步。

中 System.Collections.Generic 集合的默认实现不会同步。

此方法是 O (1) 操作。

适用于

Queue<T>.GetEnumerator 方法

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