The foreach statement of the C# language (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. StringEnumerator.Reset also brings the enumerator back to this position. At this position, calling StringEnumerator.Current throws an exception. Therefore, you must call StringEnumerator.MoveNext to advance the enumerator to the first element of the collection before reading the value of StringEnumerator.Current.

StringEnumerator.Current returns the same object until either StringEnumerator.MoveNext or StringEnumerator.Reset is called. StringEnumerator.MoveNext sets StringEnumerator.Current to the next element.

If StringEnumerator.MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and StringEnumerator.MoveNext returns false. When the enumerator is at this position, subsequent calls to StringEnumerator.MoveNext also return false. If the last call to StringEnumerator.MoveNext returned false, calling StringEnumerator.Current throws an exception. To set StringEnumerator.Current to the first element of the collection again, you can call StringEnumerator.Reset followed by StringEnumerator.MoveNext.

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 the next call to StringEnumerator.MoveNext or StringEnumerator.Reset throws an InvalidOperationException. If the collection is modified between StringEnumerator.MoveNext and StringEnumerator.Current, StringEnumerator.Current returns the element that it is set to, even if the enumerator is already invalidated.

The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can still modify the collection, which causes the enumerator to throw an exception. To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.

Namespace: System.Collections.Specialized
Assembly: System (in System.dll)
Assembly Versions: 1.0.3300.0, 1.0.5000.0, 2.0.0.0, 4.0.0.0