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DictionaryBase.OnValidate Method

Performs additional custom processes when validating the element with the specified key and value.

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

'Declaration
Protected Overridable Sub OnValidate ( _
	key As Object, _
	value As Object _
)

Parameters

key
Type: System.Object

The key of the element to validate.

value
Type: System.Object

The value of the element to validate.

The default implementation of this method is intended to be overridden by a derived class to perform some action when the specified element is validated.

The On* methods are invoked only on the instance returned by the Dictionary property, but not on the instance returned by the InnerHashtable property.

The default implementation of this method is an O(1) operation.

Notes to Implementers

This method allows implementers to define processes that must be performed when executing the standard behavior of the underlying System.Collections.Hashtable. By defining this method, implementers can add functionality to inherited methods without having to override all other methods.

OnValidate can be used to impose restrictions on the type of objects that are accepted into the collection. The default implementation prevents Nothing from being added to or removed from the underlying System.Collections.Hashtable.

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

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

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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