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DictionaryBase Class

Provides the abstract base class for a strongly typed collection of key/value pairs.

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

'Declaration
<SerializableAttribute> _
<ComVisibleAttribute(True)> _
Public MustInherit Class DictionaryBase _
	Implements IDictionary, ICollection, IEnumerable
'Usage
Dim instance As DictionaryBase

The foreach statement of the C# language (for each in Visual Basic) requires the type of each element in the collection. Since each element of the DictionaryBase is a key/value pair, the element type is not the type of the key or the type of the value. Instead, the element type is DictionaryEntry.

The foreach statement is a wrapper around the enumerator, which only allows reading from, not writing to, the collection.

NoteNote:

Because keys can be inherited and their behavior changed, their absolute uniqueness cannot be guaranteed by comparisons using the Equals method.

Notes to Implementers:

This base class is provided to make it easier for implementers to create a strongly typed custom collection. Implementers are encouraged to extend this base class instead of creating their own.

Members of this base class are protected and are intended to be used through a derived class only.

The following code example implements the DictionaryBase class and uses that implementation to create a dictionary of String keys and values that have a Length of 5 characters or less.

Imports System
Imports System.Collections

Public Class ShortStringDictionary
   Inherits DictionaryBase

   Default Public Property Item(key As String) As String 
      Get 
         Return CType(Dictionary(key), String)
      End Get 
      Set
         Dictionary(key) = value
      End Set 
   End Property 

   Public ReadOnly Property Keys() As ICollection
      Get 
         Return Dictionary.Keys
      End Get 
   End Property 

   Public ReadOnly Property Values() As ICollection
      Get 
         Return Dictionary.Values
      End Get 
   End Property 

   Public Sub Add(key As String, value As String)
      Dictionary.Add(key, value)
   End Sub 'Add

   Public Function Contains(key As String) As Boolean 
      Return Dictionary.Contains(key)
   End Function 'Contains

   Public Sub Remove(key As String)
      Dictionary.Remove(key)
   End Sub 'Remove

   Protected Overrides Sub OnInsert(key As Object, value As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If  
      If Not GetType(System.String).IsAssignableFrom(value.GetType()) Then 
         Throw New ArgumentException("value must be of type String.", "value")
      Else 
         Dim strValue As String = CType(value, String)
         If strValue.Length > 5 Then 
            Throw New ArgumentException("value must be no more than 5 characters in length.", "value")
         End If 
      End If 
   End Sub 'OnInsert

   Protected Overrides Sub OnRemove(key As Object, value As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If 
   End Sub 'OnRemove

   Protected Overrides Sub OnSet(key As Object, oldValue As Object, newValue As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If  
      If Not GetType(System.String).IsAssignableFrom(newValue.GetType()) Then 
         Throw New ArgumentException("newValue must be of type String.", "newValue")
      Else 
         Dim strValue As String = CType(newValue, String)
         If strValue.Length > 5 Then 
            Throw New ArgumentException("newValue must be no more than 5 characters in length.", "newValue")
         End If 
      End If 
   End Sub 'OnSet

   Protected Overrides Sub OnValidate(key As Object, value As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If  
      If Not GetType(System.String).IsAssignableFrom(value.GetType()) Then 
         Throw New ArgumentException("value must be of type String.", "value")
      Else 
         Dim strValue As String = CType(value, String)
         If strValue.Length > 5 Then 
            Throw New ArgumentException("value must be no more than 5 characters in length.", "value")
         End If 
      End If 
   End Sub 'OnValidate 

End Class 'ShortStringDictionary


Public Class SamplesDictionaryBase

   Public Shared Sub Main()

      ' Creates and initializes a new DictionaryBase. 
      Dim mySSC As New ShortStringDictionary()

      ' Adds elements to the collection.
      mySSC.Add("One", "a")
      mySSC.Add("Two", "ab")
      mySSC.Add("Three", "abc")
      mySSC.Add("Four", "abcd")
      mySSC.Add("Five", "abcde")

      ' Display the contents of the collection using For Each. This is the preferred method.
      Console.WriteLine("Contents of the collection (using For Each):")
      PrintKeysAndValues1(mySSC)

      ' Display the contents of the collection using the enumerator.
      Console.WriteLine("Contents of the collection (using enumerator):")
      PrintKeysAndValues2(mySSC)

      ' Display the contents of the collection using the Keys property and the Item property.
      Console.WriteLine("Initial contents of the collection (using Keys and Item):")
      PrintKeysAndValues3(mySSC)

      ' Tries to add a value that is too long. 
      Try
          mySSC.Add("Ten", "abcdefghij")
      Catch e As ArgumentException
          Console.WriteLine(e.ToString())
      End Try 

      ' Tries to add a key that is too long. 
      Try
          mySSC.Add("Eleven", "ijk")
      Catch e As ArgumentException
          Console.WriteLine(e.ToString())
      End Try

      Console.WriteLine()

      ' Searches the collection with Contains.
      Console.WriteLine("Contains ""Three"": {0}", mySSC.Contains("Three"))
      Console.WriteLine("Contains ""Twelve"": {0}", mySSC.Contains("Twelve"))
      Console.WriteLine()

      ' Removes an element from the collection.
      mySSC.Remove("Two")

      ' Displays the contents of the collection.
      Console.WriteLine("After removing ""Two"":")
      PrintKeysAndValues1(mySSC)

    End Sub 'Main


    ' Uses the For Each statement which hides the complexity of the enumerator. 
    ' NOTE: The For Each statement is the preferred way of enumerating the contents of a collection. 
    Public Shared Sub PrintKeysAndValues1(myCol As ShortStringDictionary)
      Dim myDE As DictionaryEntry
      For Each myDE In  myCol
          Console.WriteLine("   {0,-5} : {1}", myDE.Key, myDE.Value)
      Next myDE
      Console.WriteLine()
    End Sub 'PrintKeysAndValues1


    ' Uses the enumerator.  
    ' NOTE: The For Each statement is the preferred way of enumerating the contents of a collection. 
    Public Shared Sub PrintKeysAndValues2(myCol As ShortStringDictionary)
      Dim myDE As DictionaryEntry
      Dim myEnumerator As System.Collections.IEnumerator = myCol.GetEnumerator()
      While myEnumerator.MoveNext()
          If Not (myEnumerator.Current Is Nothing) Then
            myDE = CType(myEnumerator.Current, DictionaryEntry)
            Console.WriteLine("   {0,-5} : {1}", myDE.Key, myDE.Value)
          End If 
      End While
      Console.WriteLine()
    End Sub 'PrintKeysAndValues2


    ' Uses the Keys property and the Item property. 
    Public Shared Sub PrintKeysAndValues3(myCol As ShortStringDictionary)
      Dim myKeys As ICollection = myCol.Keys
      Dim k As String 
      For Each k In  myKeys
          Console.WriteLine("   {0,-5} : {1}", k, myCol(k))
      Next k
      Console.WriteLine()
    End Sub 'PrintKeysAndValues3

End Class 'SamplesDictionaryBase 


'This code produces the following output. 

'Contents of the collection (using For Each): 
'   Three : abc 
'   Five  : abcde 
'   Two   : ab 
'   One   : a 
'   Four  : abcd 

'Contents of the collection (using enumerator): 
'   Three : abc 
'   Five  : abcde 
'   Two   : ab 
'   One   : a 
'   Four  : abcd 

'Initial contents of the collection (using Keys and Item): 
'   Three : abc 
'   Five  : abcde 
'   Two   : ab 
'   One   : a 
'   Four  : abcd 

'System.ArgumentException: value must be no more than 5 characters in length. 
'Parameter name: value 
'   at ShortStringDictionary.OnValidate(Object key, Object value) 
'   at System.Collections.DictionaryBase.System.Collections.IDictionary.Add(Object key, Object value) 
'   at SamplesDictionaryBase.Main() 
'System.ArgumentException: key must be no more than 5 characters in length. 
'Parameter name: key 
'   at ShortStringDictionary.OnValidate(Object key, Object value) 
'   at System.Collections.DictionaryBase.System.Collections.IDictionary.Add(Object key, Object value) 
'   at SamplesDictionaryBase.Main() 

'Contains "Three": True 
'Contains "Twelve": False 

'After removing "Two": 
'   Three : abc 
'   Five  : abcde 
'   One   : a 
'   Four  : abcd

Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

This implementation does not provide a synchronized (thread-safe) wrapper for a DictionaryBase, but derived classes can create their own synchronized versions of the DictionaryBase using the SyncRoot property.

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.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

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

.NET Framework

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

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