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)

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 Example
   Public Name As String

   Public Sub New(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 Example instance.
Public Class ExampleInfo
   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 TestExample

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

   Public Sub Main()
      Dim ex1 As New Example("ex1")
      Dim ex2 As New Example("ex2")
      Dim ex3 As New Example("ex3")

      Dim exInfo1 As New ExampleInfo() 
      exInfo1.Name = ex1.ToString()
      exInfo1.Methods = ex1.GetType().GetMethods(flags).Count
      exInfo1.Properties = ex1.GetType().GetProperties(flags).Count

      Dim exInfo3 As New ExampleInfo() 
      exInfo3.Name = ex3.ToString()
      exInfo3.Methods = ex3.GetType().GetMethods(flags).Count
      exInfo3.Properties = ex3.GetType().GetProperties(flags).Count

      Dim attached As New ConditionalWeakTable(Of Example, ExampleInfo)
      Dim value As ExampleInfo = Nothing

      ' Attach a property to ex1 using the Add method, then retrieve it.
      attached.Add(ex1, exInfo1)
      If attached.TryGetValue(ex1, value) Then
         Console.WriteLine("{0}, {1}", ex1, value)
      Else
         Console.WriteLine("{0} does not have an attached property.", ex1)
      End If

      ' Attempt to retrieve the value attached to ex2.
      value = attached.GetValue(ex2, AddressOf TestExample.CreateAttachedValue)      
      If attached.TryGetValue(ex2, value) Then
         Console.WriteLine("{0}, {1}", ex2, value)
      Else 
         Console.WriteLine("{0} does not have an attached property.", ex2)
      End If

      ' Atttempt to retrieve the value attached to ex3.
      value = attached.GetOrCreateValue(ex3)
      Console.WriteLine("{0}, {1}", ex3, value)
   End Sub

   Public Function CreateAttachedValue(ex As Example) As ExampleInfo
      Dim info As New ExampleInfo()
      info.Name = ex.ToString()
      info.Methods = ex.GetType().GetMethods(flags).Count
      info.Properties = ex.GetType().GetProperties(flags).Count
      Return info
   End Function
End Module
' The example displays the following output:
'       ex1, ex1: 4 Methods, 0 Properties
'       ex2, ex2: 4 Methods, 0 Properties
'       ex3, : 0 Methods, 0 Properties

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 4.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
Return to top
Show: