IEnumerator Interface
Supports a simple iteration over a non-generic collection.
Assembly: mscorlib (in mscorlib.dll)
| Name | Description | |
|---|---|---|
![]() | Current | Gets the element in the collection at the current position of the enumerator. |
| Name | Description | |
|---|---|---|
![]() | MoveNext() | Advances the enumerator to the next element of the collection. |
![]() | Reset() | Sets the enumerator to its initial position, which is before the first element in the collection. |
IEnumerator is the base interface for all non-generic enumerators.
For the generic version of this interface see IEnumerator(Of T).
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.
The Reset method is provided for COM interoperability and does not need to be fully implemented; instead, the implementer can throw a NotSupportedException.
Initially, the enumerator is positioned before the first element in the collection. You must call the MoveNext method to advance the enumerator to the first element of the collection before reading the value of Current; otherwise, Current is undefined.
Current returns the same object until either MoveNext or Reset is called. MoveNext sets Current to the next element.
If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, calling Current throws an exception.
To set Current to the first element of the collection again, you can call Reset, if it’s implemented, followed by MoveNext. If Reset is not implemented, you must create a new enumerator instance to return to the first element of the collection.
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 MoveNext or Reset throws an InvalidOperationException. If the collection is modified between MoveNext and Current, 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.
The following code example demonstrates the implementation of the IEnumerable and IEnumerator interfaces for a custom collection. In this example, members of these interfaces are not explicitly called, but they are implemented to support the use of foreach (for each in Visual Basic) to iterate through the collection.
Imports System Imports System.Collections ' Simple business object. Public Class Person Public Sub New(ByVal fName As String, ByVal lName As String) Me.firstName = fName Me.lastName = lName End Sub Public firstName As String Public lastName As String End Class ' Collection of Person objects, which implements IEnumerable so that ' it can be used with ForEach syntax. Public Class People Implements IEnumerable Private _people() As Person Public Sub New(ByVal pArray() As Person) _people = New Person(pArray.Length - 1) {} Dim i As Integer For i = 0 To pArray.Length - 1 _people(i) = pArray(i) Next i End Sub ' Implementation of GetEnumerator. Public Function GetEnumerator() As IEnumerator _ Implements IEnumerable.GetEnumerator Return New PeopleEnum(_people) End Function End Class ' When you implement IEnumerable, you must also implement IEnumerator. Public Class PeopleEnum Implements IEnumerator Public _people() As Person ' Enumerators are positioned before the first element ' until the first MoveNext() call. Dim position As Integer = -1 Public Sub New(ByVal list() As Person) _people = list End Sub Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext position = position + 1 Return (position < _people.Length) End Function Public Sub Reset() Implements IEnumerator.Reset position = -1 End Sub Public ReadOnly Property Current() As Object Implements IEnumerator.Current Get Try Return _people(position) Catch ex As IndexOutOfRangeException Throw New InvalidOperationException() End Try End Get End Property End Class Class App Shared Sub Main() Dim peopleArray() As Person = { _ New Person("John", "Smith"), _ New Person("Jim", "Johnson"), _ New Person("Sue", "Rabon")} Dim peopleList As New People(peopleArray) Dim p As Person For Each p In peopleList Console.WriteLine(p.firstName + " " + p.lastName) Next End Sub End Class ' This code produces output similar to the following: ' ' John Smith ' Jim Johnson ' Sue Rabon
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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