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ConditionalWeakTable(TKey, TValue).CreateValueCallback Delegate

ConditionalWeakTable(Of TKey, TValue).CreateValueCallback Delegate

Represents a method that creates a non-default value to add as part of a key/value pair to a ConditionalWeakTable(Of TKey, TValue) object.

Namespace:  System.Runtime.CompilerServices
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
Public Delegate Function CreateValueCallback ( _
	key As TKey _
) As TValue

Parameters

key
Type: TKey
The key that belongs to the value to create.

Return Value

Type: TValue
An instance of a reference type that represents the value to attach to the specified key.

The ConditionalWeakTable(Of TKey, TValue).CreateValueCallback delegate encapsulates a callback method that is invoked by the ConditionalWeakTable(Of TKey, TValue).GetValue method when the key passed to the method is not found in the ConditionalWeakTable(Of TKey, TValue) object. The GetValue method passes the callback method the key that represents a managed object to which a property value is to be dynamically attached. The method is responsible for returning the property value to its caller, which, in turn, adds the key/value pair to the table.

The ConditionalWeakTable(Of TKey, TValue).CreateValueCallback method is used to return an instance of a reference type that can be attached to the specified key. It enables that instance to be initialized using non-default values. If the key is not found in the table, the ConditionalWeakTable(Of TKey, TValue).GetOrCreateValue method adds a key/value pair in which the value is initialized using default values.

The following example defines a MainClass class and a MainInfo class. MainInfo provides information about the MainClass instance. It also defines a static (Shared in Visual Basic) CreateAttachedValue method that can be assigned to the ConditionalWeakTable(Of TKey, TValue).CreateValueCallback delegate and passed to the GetValue method. The example calls the GetValue method to add a MainClass object and its corresponding MainInfo object to a ConditionalWeakTable(Of TKey, TValue) table. The example also illustrates calls to the Add and GetOrCreateValue methods to add key/value pairs to the table, and a call to the TryGetValue method to retrieve the value that belongs to an existing key.


Imports System.Reflection
Imports System.Runtime.CompilerServices

Public Class MainClass
   Public Name As String

   Public Sub New(ByVal name As String)
      Me.Name = name
   End Sub

   Public Overrides Function ToString() As String
      Return Me.Name
   End Function
End Class

' Define a class to contain information about each MainClass instance.
Public Class MainInfo
   Public Name As String
   Public Methods As Integer
   Public Properties As Integer

   Public Overrides Function ToString() As String
      Return String.Format("{0}: {1} Methods, {2} Properties", _
                           Me.Name, Me.Methods, Me.Properties)
   End Function
End Class

Module Example
   Private flags As BindingFlags = BindingFlags.Public Or BindingFlags.Instance

   Public Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)
      Dim m1 As New MainClass("m1")
      Dim m2 As New MainClass("m2")
      Dim m3 As New MainClass("m3")

      Dim mInfo1 As New MainInfo()
      mInfo1.Name = m1.ToString()
      mInfo1.Methods = m1.GetType().GetMethods(flags).Count
      mInfo1.Properties = m1.GetType().GetProperties(flags).Count

      Dim mInfo3 As New MainInfo()
      mInfo3.Name = m3.ToString()
      mInfo3.Methods = m3.GetType().GetMethods(flags).Count
      mInfo3.Properties = m3.GetType().GetProperties(flags).Count

      Dim attached As New ConditionalWeakTable(Of MainClass, MainInfo)
      Dim value As MainInfo = Nothing

      ' Attach a property to m1 using the Add method, then retrieve it.
      attached.Add(m1, mInfo1)
      If attached.TryGetValue(m1, value) Then
         outputBlock.Text += String.Format("{0}, {1}", m1, value) & vbCrLf
      Else
         outputBlock.Text += String.Format("{0} does not have an attached property.", m1) & vbCrLf
      End If

      ' Attempt to retrieve the value attached to m2.
      value = attached.GetValue(m2, AddressOf Example.CreateAttachedValue)
      If attached.TryGetValue(m2, value) Then
         outputBlock.Text += String.Format("{0}, {1}", m2, value) & vbCrLf
      Else
         outputBlock.Text += String.Format("{0} does not have an attached property.", m2) & vbCrLf
      End If

      ' Atttempt to retrieve the value attached to m3.
      value = attached.GetOrCreateValue(m3)
      outputBlock.Text += String.Format("{0}, {1}", m3, value) & vbCrLf
   End Sub

   Public Function CreateAttachedValue(ByVal m As MainClass) As MainInfo
      Dim info As New MainInfo()
      info.Name = m.ToString()
      info.Methods = m.GetType().GetMethods(flags).Count
      info.Properties = m.GetType().GetProperties(flags).Count
      Return info
   End Function
End Module
' The example displays the following output:
'       m1, m1: 4 Methods, 0 Properties
'       m2, m2: 4 Methods, 0 Properties
'       m3, : 0 Methods, 0 Properties


Silverlight

Supported in: 5, 4

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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