IDictionaryEnumerator Interface
Enumerates the elements of a nongeneric dictionary.
Assembly: mscorlib (in mscorlib.dll)
[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
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