IEnumerator(T) Interface
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IEnumerator(Of T) Interface

 

Supports a simple iteration over a generic collection.

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

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

Type Parameters

OutT

The type of objects to enumerate.

NameDescription
System_CAPS_pubpropertyCurrent

Gets the element in the collection at the current position of the enumerator.

NameDescription
System_CAPS_pubmethodDispose()

Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.(Inherited from IDisposable.)

System_CAPS_pubmethodMoveNext()

Advances the enumerator to the next element of the collection.(Inherited from IEnumerator.)

System_CAPS_pubmethodReset()

Sets the enumerator to its initial position, which is before the first element in the collection.(Inherited from IEnumerator.)

IEnumerator(Of 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. However, if you choose to do this, you should make sure no callers are relying on the Reset functionality.

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.

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 Resetmethods 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.

In addition, IEnumerator(Of T) implements IDisposable, which requires you to implement the Dispose method. This enables you to close database connections or release file handles or similar operations when using other resources. If there are no additional resources to dispose of, provide an empty Dispose implementation.

The following example shows an implementation of the IEnumerator(Of 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(Of T) interface.

' 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

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 2.0
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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