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IDictionaryEnumerator Interface

Enumerates the elements of a nongeneric dictionary.

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

'Declaration
<ComVisibleAttribute(True)> _
Public Interface IDictionaryEnumerator _
	Inherits IEnumerator

The IDictionaryEnumerator type exposes the following members.

  NameDescription
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsCurrentGets the current element in the collection. (Inherited from IEnumerator.)
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsEntryGets both the key and the value of the current dictionary entry.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsKeyGets the key of the current dictionary entry.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsValueGets the value of the current dictionary entry.
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  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsMoveNextAdvances the enumerator to the next element of the collection. (Inherited from IEnumerator.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsResetSets the enumerator to its initial position, which is before the first element in the collection. (Inherited from IEnumerator.)
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[Visual Basic, C#]

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.

Notes to Implementers

The Current property that is inherited from IEnumerator returns an Object that is a boxed DictionaryEntry, similar to the return value of the Entry property.

This code example shows how to define a dictionary enumerator that implements the IDictionaryEnumerator interface.

Imports System
Imports System.Collections

' This class implements a simple dictionary using an array of DictionaryEntry objects (key/value pairs). 
Public Class SimpleDictionary
    Implements IDictionary

    ' The array of items 
    Dim items() As DictionaryEntry
    Dim ItemsInUse As Integer = 0

    ' Construct the SimpleDictionary with the desired number of items. 
    ' The number of items cannot change for the life time of this SimpleDictionary. 
    Public Sub New(ByVal numItems As Integer)
        items = New DictionaryEntry(numItems - 1) {}
    End Sub 

    ' IDictionary Members 
    Public ReadOnly Property IsReadOnly() As Boolean Implements IDictionary.IsReadOnly
        Get 
            Return False 
        End Get 
    End Property 

    Public Function Contains(ByVal key As Object) As Boolean Implements IDictionary.Contains
        Dim index As Integer 
        Return TryGetIndexOfKey(key, index)
    End Function 

    Public ReadOnly Property IsFixedSize() As Boolean Implements IDictionary.IsFixedSize
        Get 
            Return False 
        End Get 
    End Property 

    Public Sub Remove(ByVal key As Object) Implements IDictionary.Remove
        If key = Nothing Then 
            Throw New ArgumentNullException("key")
        End If 
        ' Try to find the key in the DictionaryEntry array 
        Dim index As Integer 
        If TryGetIndexOfKey(key, index) Then 

            ' If the key is found, slide all the items up.
            Array.Copy(items, index + 1, items, index, (ItemsInUse - index) - 1)
            ItemsInUse = ItemsInUse - 1
        Else 

            ' If the key is not in the dictionary, just return.  
        End If 
    End Sub 

    Public Sub Clear() Implements IDictionary.Clear
        ItemsInUse = 0
    End Sub 

    Public Sub Add(ByVal key As Object, ByVal value As Object) Implements IDictionary.Add

        ' Add the new key/value pair even if this key already exists in the dictionary. 
        If ItemsInUse = items.Length Then 
            Throw New InvalidOperationException("The dictionary cannot hold any more items.")
        End If
        items(ItemsInUse) = New DictionaryEntry(key, value)
        ItemsInUse = ItemsInUse + 1
    End Sub 

    Public ReadOnly Property Keys() As ICollection Implements IDictionary.Keys
        Get 

            ' Return an array where each item is a key. 
            ' Note: Declaring keyArray() to have a size of ItemsInUse - 1 
            '       ensures that the array is properly sized, in VB.NET 
            '       declaring an array of size N creates an array with 
            '       0 through N elements, including N, as opposed to N - 1 
            '       which is the default behavior in C# and C++. 
            Dim keyArray() As Object = New Object(ItemsInUse - 1) {}
            Dim n As Integer 
            For n = 0 To ItemsInUse - 1
                keyArray(n) = items(n).Key
            Next n

            Return keyArray
        End Get 
    End Property 

    Public ReadOnly Property Values() As ICollection Implements IDictionary.Values
        Get 
            ' Return an array where each item is a value. 
            Dim valueArray() As Object = New Object(ItemsInUse - 1) {}
            Dim n As Integer 
            For n = 0 To ItemsInUse - 1
                valueArray(n) = items(n).Value
            Next n

            Return valueArray
        End Get 
    End Property 

    Public Property Item(ByVal key As Object) As Object Implements IDictionary.Item
        Get 

            ' If this key is in the dictionary, return its value. 
            Dim index As Integer 
            If TryGetIndexOfKey(key, index) Then 

                ' The key was found return its value. 
                Return items(index).Value
            Else 

                ' The key was not found return null. 
                Return Nothing 
            End If 
        End Get 

        Set(ByVal value As Object)
            ' If this key is in the dictionary, change its value.  
            Dim index As Integer 
            If TryGetIndexOfKey(key, index) Then 

                ' The key was found change its value.
                items(index).Value = value
            Else 

                ' This key is not in the dictionary add this key/value pair.
                Add(key, value)
            End If 
        End Set 
    End Property 

    Private Function TryGetIndexOfKey(ByVal key As Object, ByRef index As Integer) As Boolean 
        For index = 0 To ItemsInUse - 1
            ' If the key is found, return true (the index is also returned). 
            If items(index).Key.Equals(key) Then 
                Return True 
            End If 
        Next index

        ' Key not found, return false (index should be ignored by the caller). 
        Return False 
    End Function 

    Private Class SimpleDictionaryEnumerator
        Implements IDictionaryEnumerator

        ' A copy of the SimpleDictionary object's key/value pairs. 
        Dim items() As DictionaryEntry
        Dim index As Integer = -1

        Public Sub New(ByVal sd As SimpleDictionary)
            ' Make a copy of the dictionary entries currently in the SimpleDictionary object.
            items = New DictionaryEntry(sd.Count - 1) {}
            Array.Copy(sd.items, 0, items, 0, sd.Count)
        End Sub 

        ' Return the current item. 
        Public ReadOnly Property Current() As Object Implements IDictionaryEnumerator.Current
            Get
                ValidateIndex()
                Return items(index)
            End Get 
        End Property 

        ' Return the current dictionary entry. 
        Public ReadOnly Property Entry() As DictionaryEntry Implements IDictionaryEnumerator.Entry
            Get 
                Return Current
            End Get 
        End Property 

        ' Return the key of the current item. 
        Public ReadOnly Property Key() As Object Implements IDictionaryEnumerator.Key
            Get
                ValidateIndex()
                Return items(index).Key
            End Get 
        End Property 

        ' Return the value of the current item. 
        Public ReadOnly Property Value() As Object Implements IDictionaryEnumerator.Value
            Get
                ValidateIndex()
                Return items(index).Value
            End Get 
        End Property 

        ' Advance to the next item. 
        Public Function MoveNext() As Boolean Implements IDictionaryEnumerator.MoveNext
            If index < items.Length - 1 Then
                index = index + 1
                Return True 
            End If 

            Return False 
        End Function 

        ' Validate the enumeration index and throw an exception if the index is out of range. 
        Private Sub ValidateIndex()
            If index < 0 Or index >= items.Length Then 
                Throw New InvalidOperationException("Enumerator is before or after the collection.")
            End If 
        End Sub 

        ' Reset the index to restart the enumeration. 
        Public Sub Reset() Implements IDictionaryEnumerator.Reset
            index = -1
        End Sub 

    End Class 

    Public Function GetEnumerator() As IDictionaryEnumerator Implements IDictionary.GetEnumerator

        'Construct and return an enumerator. 
        Return New SimpleDictionaryEnumerator(Me)
    End Function 


    ' ICollection Members 
    Public ReadOnly Property IsSynchronized() As Boolean Implements IDictionary.IsSynchronized
        Get 
            Return False 
        End Get 
    End Property 

    Public ReadOnly Property SyncRoot() As Object Implements IDictionary.SyncRoot
        Get 
            Throw New NotImplementedException()
        End Get 
    End Property 

    Public ReadOnly Property Count() As Integer Implements IDictionary.Count
        Get 
            Return ItemsInUse
        End Get 
    End Property 

    Public Sub CopyTo(ByVal array As Array, ByVal index As Integer) Implements IDictionary.CopyTo
        Throw New NotImplementedException()
    End Sub 

    ' IEnumerable Members 
    Public Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator

        ' Construct and return an enumerator. 
        Return Me.GetEnumerator()
    End Function 
End Class 

Public NotInheritable Class App
    Public Shared Sub Main()
        ' Create a dictionary that contains no more than three entries. 
        Dim d As IDictionary = New SimpleDictionary(3)

        ' Add three people and their ages to the dictionary.
        d.Add("Jeff", 40)
        d.Add("Kristin", 34)
        d.Add("Aidan", 1)

        Console.WriteLine("Number of elements in dictionary = {0}", d.Count)

        Console.WriteLine("Does dictionary contain 'Jeff'? {0}", d.Contains("Jeff"))
        Console.WriteLine("Jeff's age is {0}", d("Jeff"))

        ' Display every entry's key and value. 
        Dim de As DictionaryEntry
        For Each de In d
            Console.WriteLine("{0} is {1} years old.", de.Key, de.Value)
        Next 

        ' Remove an entry that exists.
        d.Remove("Jeff")

        ' Remove an entry that does not exist, but do not throw an exception.
        d.Remove("Max")

        ' Show the names (keys) of the people in the dictionary. 
        Dim s As String 

        For Each s In d.Keys
            Console.WriteLine(s)
        Next 

        ' Show the ages (values) of the people in the dictionary. 
        Dim age As Integer 
        For Each age In d.Values
            Console.WriteLine(age)
        Next 

    End Sub 
End Class 

' This code produces the following output. 

' Number of elements in dictionary = 3 
' Does dictionary contain 'Jeff'? True 
' Jeff's age is 40 
' Jeff is 40 years old. 
' Kristin is 34 years old. 
' Aidan is 1 years old. 
' Kristin 
' Aidan 
' 34 
' 1

.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.

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