Article
11/18/2011

IEnumerator Interface

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Supports a simple iteration over a nongeneric collection.

Namespace: System.Collections
Assembly: mscorlib (in mscorlib.dll)

Syntax

'Declaration
<GuidAttribute("496B0ABF-CDEE-11d3-88E8-00902754C43A")> _
<ComVisibleAttribute(True)> _
Public Interface IEnumerator

[GuidAttribute("496B0ABF-CDEE-11d3-88E8-00902754C43A")]
[ComVisibleAttribute(true)]
public interface IEnumerator

The IEnumerator type exposes the following members.

Properties

	Name	Description
	Current	Gets the current element in the collection.

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Methods

	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.

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Remarks

IEnumerator is the base interface for all nongeneric enumerators.

For the generic version of this interface see IEnumerator<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.

Initially, the enumerator is positioned before the first element in the collection. The Reset method also brings the enumerator back to this position. At this position, calling the Current property throws an exception. Therefore, you must call the MoveNext method to advance the enumerator to the first element of the collection before reading the value of Current.

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 followed by 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 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.

Examples

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.

Note:
To run this example, see Building Examples That Use a Demo Method and a TextBlock Control.

Imports System.Collections

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

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

   Public Function GetEnumerator() As IEnumerator _
     Implements IEnumerable.GetEnumerator

      Return New PeopleEnum(_people)
   End Function

End Class

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 Example
   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      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
         outputBlock.Text &= p.firstName + " " + p.lastName & vbCrLf
      Next

   End Sub
End Class

' This code produces output similar to the following:
' 
' John Smith
' Jim Johnson
' Sue Rabon

using System;
using System.Collections;

public class Person
{
   public Person(string fName, string lName)
   {
      this.firstName = fName;
      this.lastName = lName;
   }

   public string firstName;
   public string lastName;
}

public class People : IEnumerable
{
   private Person[] _people;
   public People(Person[] pArray)
   {
      _people = new Person[pArray.Length];

      for (int i = 0; i < pArray.Length; i++)
      {
         _people[i] = pArray[i];
      }
   }

   public IEnumerator GetEnumerator()
   {
      return new PeopleEnum(_people);
   }
}

public class PeopleEnum : IEnumerator
{
   public Person[] _people;

   // Enumerators are positioned before the first element
   // until the first MoveNext() call.
   int position = -1;

   public PeopleEnum(Person[] list)
   {
      _people = list;
   }

   public bool MoveNext()
   {
      position++;
      return (position < _people.Length);
   }

   public void Reset()
   {
      position = -1;
   }

   public object Current
   {
      get
      {
         try
         {
            return _people[position];
         }
         catch (IndexOutOfRangeException)
         {
            throw new InvalidOperationException();
         }
      }
   }
}

class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      Person[] peopleArray = new Person[3]
        {
            new Person("John", "Smith"),
            new Person("Jim", "Johnson"),
            new Person("Sue", "Rabon"),
        };

      People peopleList = new People(peopleArray);
      foreach (Person p in peopleList)
         outputBlock.Text += p.firstName + " " + p.lastName + "\n";

   }
}

/* This code produces output similar to the following:
 * 
 * John Smith
 * Jim Johnson
 * Sue Rabon
 * 
 */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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