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.

The dictionary is maintained in a sorted order using an internal tree. Every new element is positioned at the correct sort position, and the tree is adjusted to maintain the sort order whenever an element is removed. While enumerating, the sort order is maintained.

Initially, the enumerator is positioned before the first element in the collection. At this position, IEnumerator`1.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`1.Current.

IEnumerator`1.Current returns the same object until IEnumerator.MoveNext is called. IEnumerator.MoveNext sets IEnumerator`1.Current to the next element.

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 return false. If the last call to IEnumerator.MoveNext returned false, IEnumerator`1.Current is undefined. You cannot set IEnumerator`1.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.

Namespace: System.Collections.Generic
Assembly: System (in System.dll)
Assembly Versions: 2.0.0.0, 4.0.0.0
Since: .NET 2.0