IEnumerator(Of T) Interface
Supports a simple iteration over a generic collection.
Assembly: mscorlib (in mscorlib.dll)
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(Of T) type exposes the following members.
| Name | Description | |
|---|---|---|
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Current | Gets the current element in the collection. (Inherited from IEnumerator.) |
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Current | Gets the element in the collection at the current position of the enumerator. |
| Name | Description | |
|---|---|---|
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Dispose | Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources. (Inherited from IDisposable.) |
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MoveNext | Advances the enumerator to the next element of the collection. (Inherited from IEnumerator.) |
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Reset | 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.
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.
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 : 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; } } }
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.
''' <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
- 7/8/2011
- Omar Amin Ibrahim
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.
- 5/4/2011
- ChrisTrout