ConditionalWeakTable(Of TKey, TValue).GetOrCreateValue Method (TKey)

 

Atomically searches for a specified key in the table and returns the corresponding value. If the key does not exist in the table, the method invokes the default constructor of the class that represents the table's value to create a value that is bound to the specified key.

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

Public Function GetOrCreateValue (
	key As TKey
) As TValue

Parameters

key
Type: TKey

The key to search for. key represents the object to which the property is attached.

Return Value

Type: TValue

The value that corresponds to key, if key already exists in the table; otherwise, a new value created by the default constructor of the class defined by the TValue generic type parameter.

Exception Condition
ArgumentNullException

key is null.

MissingMethodException
System_CAPS_noteNote

In the .NET for Windows Store apps or the Portable Class Library, catch the base class exception, MissingMemberException, instead.

The class that represents the table's value does not define a default constructor.

If key does not exist in the table, the method adds it, along with the object that is instantiated by calling the default constructor of the class defined by the TValue generic type parameter. If the TValue class has no default constructor, a MissingMethodException is thrown.

This is the recommended method of retrieving an existing value or adding a new value to the ConditionalWeakTable(Of TKey, TValue) table if the class of the table's value defines a default constructor. If it does not define a default constructor, you can instead call the GetValue method, which relies on a callback-provided method to instantiate the object representing the table's value

To retrieve the value of an existing key without adding the key/value pair if the key is not found in the table, call the TryGetValue method.

The following example defines a MainClass class and a MainInfo class, which provides information about the MainClass instance. The example calls the GetOrCreateValue method to add a MainClass object and its attached MainInfo object to a ConditionalWeakTable(Of TKey, TValue) table. The example also illustrates calls to the Add and GetValue methods to add key/value pairs to the table, and to the TryGetValue method to retrieve the value of 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
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