[Visual Basic, C#]
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. IEnumerator.Reset also brings the enumerator back to this position. At this position, IEnumerator.Current is undefined. Therefore, you must call IEnumerator.MoveNext to advance the enumerator to the first element of the collection before reading the value of IEnumerator.Current.
If IEnumerator.MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and IEnumerator.MoveNext returns false. When the enumerator is at this position, subsequent calls to IEnumerator.MoveNext also return false. If the last call to IEnumerator.MoveNext returned false, IEnumerator.Current is undefined. To set IEnumerator.Current to the first element of the collection again, you can call IEnumerator.Reset followed by IEnumerator.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 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.
This method is an O(1) operation.
Because serializing and deserializing an enumerator for a Hashtable can cause the elements to become reordered, it is not possible to continue enumeration without calling the IEnumerator.Reset method.