IEnumerator(T) Interface (System.Collections.Generic)

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.NET Framework Class Library

IEnumerator<T> Interface

Supports a simple iteration over a generic collection.

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

Syntax

Visual Basic

Public Interface IEnumerator(Of Out T) _
	Inherits IDisposable, IEnumerator

public interface IEnumerator<out T> : IDisposable, 
	IEnumerator

Visual C++

generic<typename T>
public interface class IEnumerator : IDisposable, 
	IEnumerator

type IEnumerator<'T> =  
    interface
        interface IDisposable
        interface IEnumerator
    end

Type Parameters

out T

The type of objects to enumerate.

This type parameter is covariant. That is, you can use either the type you specified or any type that is more derived. For more information about covariance and contravariance, see Covariance and Contravariance in Generics.

The IEnumerator<T> type exposes the following members.

Properties

	Name	Description
	Current	Gets the current element in the collection. (Inherited from IEnumerator.)
	Current	Gets the element in the collection at the current position of the enumerator.

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Methods

	Name	Description
	Dispose	Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources. (Inherited from IDisposable.)
	MoveNext	Advances the enumerator to the next element of the collection. (Inherited from IEnumerator.)
	Reset	Sets the enumerator to its initial position, which is before the first element in the collection. (Inherited from IEnumerator.)

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Remarks

IEnumerator<T> is the base interface for all generic enumerators.

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.

Initially, the enumerator is positioned before the first element in the collection. At this position, Current is undefined. Therefore, you must call MoveNext to advance the enumerator to the first element of the collection before reading the value of Current.

Current returns the same object until MoveNext 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, Current is undefined. You cannot set Current to the first element of the collection again; you must create a new enumerator instance instead.

The Reset method is provided for COM interoperability. It does not necessarily need to be implemented; instead, the implementer can simply throw a NotSupportedException.

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 the System.Collections.Generic namespace are not synchronized.

Notes to Implementers

Implementing this interface requires implementing the nongeneric IEnumerator interface. The MoveNext and Reset methods do not depend on T, and appear only on the nongeneric interface. The Current property appears on both interfaces, and has different return types. Implement the nongeneric IEnumerator.Current property as an explicit interface implementation. This allows any consumer of the nongeneric interface to consume the generic interface.

Examples

The following example shows an implementation of the IEnumerator<T> interface for a collection class of custom objects. The custom object is an instance of the type Box, and the collection class is BoxCollection. This code example is part of a larger example provided for the ICollection<T> interface.

Visual Basic


' Defines the enumerator for the Boxes collection.
' (Some prefer this class nested in the collection class.)
Public Class BoxEnumerator
    Implements IEnumerator(Of Box)
    Private _collection As BoxCollection
    Private curIndex As Integer
    Private curBox As Box


    Public Sub New(ByVal collection As BoxCollection)
        MyBase.New()
        _collection = collection
        curIndex = -1
        curBox = Nothing

    End Sub

    Private Property Box As Box
    Public Function MoveNext() As Boolean _
        Implements IEnumerator(Of Box).MoveNext
        curIndex = curIndex + 1
        If curIndex = _collection.Count Then
            ' Avoids going beyond the end of the collection.
            Return False
        Else
            'Set current box to next item in collection.
            curBox = _collection(curIndex)
        End If
        Return True
    End Function

    Public Sub Reset() _
        Implements IEnumerator(Of Box).Reset
        curIndex = -1
    End Sub

    Public Sub Dispose() _
        Implements IEnumerator(Of Box).Dispose

    End Sub

    Public ReadOnly Property Current() As Box _
        Implements IEnumerator(Of Box).Current

        Get
            If curBox Is Nothing Then
                Throw New InvalidOperationException()
            End If

            Return curBox
        End Get
    End Property

    Private ReadOnly Property Current1() As Object _
        Implements IEnumerator.Current

        Get
            Return Me.Current
        End Get
    End Property



End Class

' Defines two boxes as equal if they have the same dimensions.
Public Class BoxSameDimensions
    Inherits EqualityComparer(Of Box)

    Public Overrides Function Equals(ByVal b1 As Box, ByVal b2 As Box) As Boolean
        If b1.Height = b2.Height And b1.Length = b2.Length And b1.Width = b2.Width Then
            Return True
        Else
            Return False
        End If
    End Function


    Public Overrides Function GetHashCode(ByVal bx As Box) As Integer
        Dim hCode As Integer = bx.Height ^ bx.Length ^ bx.Width
        Return hCode.GetHashCode()
    End Function

End Class



// Defines the enumerator for the Boxes collection.
// (Some prefer this class nested in the collection class.)
public class BoxEnumerator : IEnumerator<Box>
{
    private BoxCollection _collection;
    private int curIndex;
    private Box curBox;


    public BoxEnumerator(BoxCollection collection)
    {
        _collection = collection;
        curIndex = -1;
        curBox = default(Box);

    }

    public bool MoveNext()
    {
        //Avoids going beyond the end of the collection.
        if (++curIndex >= _collection.Count)
        {
            return false;
        }
        else
        {
            // Set current box to next item in collection.
            curBox = _collection[curIndex];
        }
        return true;
    }

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

    void IDisposable.Dispose() { }

    public Box Current
    {
        get { return curBox; }
    }


    object IEnumerator.Current
    {
        get { return Current; }
    }

}

Version Information

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Platforms

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference

System.Collections.Generic Namespace

IEnumerable<T>

ICollection<T>

System.Collections

Community Content

genu

Incorrect implementation of Reset()

The example does not properly Reset().
If a developer were to work through half of the Enumerator and call Reset() followed by Current(), (s)he would receive the last value of curBox rather than null.

Rather than having a "curBox" implementation, why not use the index and collection?
Reset would set the Index back to -1 and Current could return default(T) if Index = -1.

Omar Amin Ibrahim

Implementation of IEnumerable(Of T) & IEnumerator(Of T)

''' <summary>
''' Copyright © Omar Amin Ibrahim 2011
''' silverlight1212@yahoo.com
''' </summary>
''' <remarks></remarks>
Public NotInheritable Class RiverNileEnumerator

    Public Shared Function GetList(Of T)(ByVal list As IList(Of T)) As RiverNileListEnumerator(Of T)
        If (list Is Nothing) Then
            Throw New ArgumentNullException("list")
        End If
        Return New RiverNileListEnumerator(Of T)(list)
    End Function

    Public Shared Function GetCollection(Of T)(ByVal collection As ICollection(Of T)) As RiverNileCollectionEnumerator(Of T)
        If (collection Is Nothing) Then
            Throw New ArgumentNullException("collection")
        End If
        Return New RiverNileCollectionEnumerator(Of T)(collection)
    End Function

    Public Class RiverNileCollectionEnumerator(Of T)
        Implements IEnumerable(Of T)
        Implements IEnumerator(Of T)

#Region " Fields "

        Private _collectiont As ICollection(Of T)
        Private _index As Integer
        Private _current As T

#End Region

#Region " Constructor "

        Public Sub New(ByVal collection As ICollection(Of T))
            MyBase.New()
            _collectiont = collection
            _index = -1
            _current = CType(Nothing, T)
        End Sub

#End Region

#Region " Properties "

        Public ReadOnly Property Current As T
            Get
                If _current Is Nothing Then
                    Throw New InvalidOperationException()
                End If

                Return _current
            End Get
        End Property

        Private ReadOnly Property System_Collections_Generic_Current As T Implements System.Collections.Generic.IEnumerator(Of T).Current
            Get
                Return Me.Current
            End Get
        End Property

        Private ReadOnly Property System_Collections_Current As Object Implements System.Collections.IEnumerator.Current
            Get
                Return Me.Current
            End Get
        End Property

#End Region

#Region " Methods "

        Public Function GetEnumerator() As IEnumerator(Of T)
            Return New RiverNileCollectionEnumerator(Of T)(Me._collectiont)
        End Function

        Public Function MoveNext() As Boolean
            _index = _index + 1
            If _index = _collectiont.Count Then
                Return False
            Else
                _current = _collectiont(_index)
            End If
            Return True
        End Function

        Public Sub Reset()
            _index = -1
            _current = CType(Nothing, T)
        End Sub

        Private Function System_Collections_Generic_GetEnumerator() As System.Collections.Generic.IEnumerator(Of T) Implements System.Collections.Generic.IEnumerable(Of T).GetEnumerator
            Return Me.GetEnumerator
        End Function

        Private Function System_Collections_GetEnumerator() As System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator
            Return Me.GetEnumerator
        End Function

        Private Function System_Collections_MoveNext() As Boolean Implements System.Collections.IEnumerator.MoveNext
            Return MoveNext()
        End Function

        Private Sub System_Collections_Reset() Implements System.Collections.IEnumerator.Reset
            Me.Reset()
        End Sub

        Private Sub System_IDisposable_Dispose() Implements IDisposable.Dispose
            ' do nothing
        End Sub

#End Region

    End Class

    Public Class RiverNileListEnumerator(Of T)

        Implements IEnumerable(Of T)
        Implements IEnumerator(Of T)

#Region " Fields "

        Private _list As IList(Of T)
        Private _index As Integer
        Private _current As T

#End Region

#Region " Constructor "

        Public Sub New(ByVal list As IList(Of T))
            MyBase.New()
            _list = list
            _index = -1
            _current = CType(Nothing, T)
        End Sub

#End Region

#Region " Properties "

        Public ReadOnly Property Current As T Implements System.Collections.Generic.IEnumerator(Of T).Current
            Get
                If _current Is Nothing Then
                    Throw New InvalidOperationException()
                End If

                Return _current
            End Get
        End Property

        Private ReadOnly Property System_Collections_Current As Object Implements System.Collections.IEnumerator.Current
            Get
                Return Me.Current
            End Get
        End Property

#End Region

#Region " Methods "

        Public Function GetEnumerator() As System.Collections.Generic.IEnumerator(Of T) Implements System.Collections.Generic.IEnumerable(Of T).GetEnumerator
            Return New RiverNileListEnumerator(Of T)(Me._list)
        End Function

        Private Function System_Collections_GetEnumerator() As System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator
            Return Me.GetEnumerator
        End Function

        Public Function MoveNext() As Boolean Implements System.Collections.IEnumerator.MoveNext
            _index = _index + 1
            If _index = _list.Count Then
                Return False
            Else
                _current = _list(_index)
            End If
            Return True
        End Function

        Public Sub Reset() Implements System.Collections.IEnumerator.Reset
            _index = -1
            _current = CType(Nothing, T)
        End Sub

        Private Sub System_IDisposable_Dispose() Implements IDisposable.Dispose
            ' do nothing
        End Sub

#End Region

    End Class

End Class ' RiverNileEnumerator


...........How To Use ...........

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Dim ColorList As ICollection(Of KnownColor) = New List(Of KnownColor) From {KnownColor.Aqua, KnownColor.Red, KnownColor.Black}

        For Each clr As KnownColor In RiverNileEnumerator.GetCollection(ColorList)
            ListBox1.Items.Add(clr)
        Next

        Dim StringList As IList(Of String) = New List(Of String) From {"Omar", "Amin", "Ibrahim"}

        For Each Str As String In RiverNileEnumerator.GetList(StringList)
            ListBox2.Items.Add(Str)
        Next
    End Sub

End Class