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)
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
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