Export (0) Print
Expand All

IEnumerable.GetEnumerator Method

Returns an enumerator that iterates through a collection.

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

'Declaration
Function GetEnumerator As IEnumerator

Return Value

Type: System.Collections.IEnumerator
An IEnumerator object that can be used to iterate through the collection.

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, the Current property is undefined. 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 returns false, Current is undefined. 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 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.

The following code example demonstrates the implementation of the IEnumerable interfaces for a custom collection. In this example, GetEnumerator is not explicitly called, but it is implemented to support the use of foreach (For Each in Visual Basic). This code example is part of a larger example for the IEnumerable interface.

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

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

Show:
© 2014 Microsoft