Dictionary<TKey,TValue>.ValueCollection.IEnumerable.GetEnumerator 方法

定义

返回循环访问集合的枚举数。Returns an enumerator that iterates through a collection.

 virtual System::Collections::IEnumerator ^ System.Collections.IEnumerable.GetEnumerator() = System::Collections::IEnumerable::GetEnumerator;
System.Collections.IEnumerator IEnumerable.GetEnumerator ();
Function GetEnumerator () As IEnumerator Implements IEnumerable.GetEnumerator

返回

可用于循环访问集合的 IEnumeratorAn IEnumerator that can be used to iterate through the collection.

实现

注解

C#for each语句( C++在VisualBasic中)隐藏了枚举器的复杂性。For Each foreachThe foreach statement of the C# language (for each in C++, For Each in Visual Basic) hides the complexity of the enumerators. 因此,建议使用 foreach,而不是直接操作枚举数。Therefore, using foreach is recommended, instead of directly manipulating the enumerator.

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。Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.

最初,枚举数定位在集合中第一个元素的前面。Initially, the enumerator is positioned before the first element in the collection. Reset 也会将枚举器放回此位置。Reset also brings the enumerator back to this position. 在此位置上,未定义 CurrentAt this position, Current is undefined. 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。Therefore, you must call MoveNext to advance the enumerator to the first element of the collection before reading the value of Current.

在调用 CurrentMoveNext 之前,Reset 返回同一对象。Current returns the same object until either MoveNext or Reset is called. MoveNextCurrent 设置为下一个元素。MoveNext sets Current to the next element.

如果MoveNext越过集合的末尾, 则枚举器将定位在集合中最后一个元素之后, 并MoveNext返回falseIf MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. 当枚举器位于此位置时, 对MoveNext的后续调用也将返回。 falseWhen the enumerator is at this position, subsequent calls to MoveNext also return false. 如果最后一次调用MoveNext返回false, Current则未定义。If the last call to MoveNext returned false, Current is undefined. 若要再次将 Current 设置为集合的第一个元素,可以调用 Reset 并接着调用 MoveNextTo set Current to the first element of the collection again, you can call Reset followed by MoveNext.

只要集合保持不变,枚举数就保持有效。An enumerator remains valid as long as the collection remains unchanged. 如果对集合所做的更改 (如添加、修改或删除元素), 则枚举器将失效且不可恢复, 并且对MoveNextReset的下InvalidOperationException一个调用将引发。If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException.

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。The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. 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。To guarantee thread safety during enumeration, you can lock the collection during the entire enumeration. 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。To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

System.Collections.Generic集合的默认实现不同步。Default implementations of collections in System.Collections.Generic are not synchronized.

此方法为 O (1) 操作。This method is an O(1) operation.

适用于

另请参阅