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WeakReference Class

Represents a weak reference, which references an object while still allowing that object to be reclaimed by garbage collection.

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

'Declaration
<SerializableAttribute> _
<ComVisibleAttribute(True)> _
<SecurityPermissionAttribute(SecurityAction.InheritanceDemand, Flags := SecurityPermissionFlag.UnmanagedCode)> _
Public Class WeakReference _
	Implements ISerializable
'Usage
Dim instance As WeakReference

A weak reference allows the garbage collector to collect an object while still allowing an application to access the object. If you need the object, you can still obtain a strong reference to it and prevent it from being collected. For more information about how to use short and long weak references, see Weak References.

The following example demonstrates how you can use weak references to maintain a cache of objects as a resource for an application. The cache is constructed using an IDictionary(Of TKey, TValue) of WeakReference objects keyed by an index value. The Target property for the WeakReference objects is an object in a byte array that represents data.

The example randomly accesses objects in the cache. If an object is reclaimed for garbage collection, a new data object is regenerated; otherwise, the object is available to access because of the weak reference.

Imports System
Imports System.Collections.Generic
Public Class Module1

    Public Shared Sub Main()

        ' Create the cache. 
        Dim cacheSize As Integer = 50
        Dim r As Random = New Random
        Dim c As Cache = New Cache(cacheSize)

        Dim DataName As String = "" 

        ' Randomly access objects in the cache. 
        Dim i As Integer
        Do While i < c.Count
            Dim index As Integer = r.Next(c.Count)

            ' Access the object by 
            ' getting a property value.
            DataName = c(index).Name

            i += 1
        Loop 

        'Show results. 
        Dim regenPercent As Double = c.RegenerationCount * 100 / c.Count
        Console.WriteLine("Cache size: {0}, Regenerated: {1}%", c.Count.ToString(), regenPercent.ToString())

    End Sub 
End Class 
Public Class Cache

    ' Dictionary to contain the cache. 
    Private Shared _cache As Dictionary(Of Integer, WeakReference)

    ' Track the number of times an 
    ' object is regenerated. 
    Dim regenCount As Integer = 0

    Public Sub New(ByVal count As Integer)
        MyBase.New()
        _cache = New Dictionary(Of Integer, WeakReference)

        ' Add data objects with a short 
        ' weak reference to the cache. 
        Dim i As Integer = 0
        Do While (i < count)
            _cache.Add(i, New WeakReference(New Data(i)))
            i = (i + 1)
        Loop 
    End Sub 

    ' Number of items in the cache. 
    Public ReadOnly Property Count() As Integer 
        Get 
            Return _cache.Count
        End Get 
    End Property 

    ' Number of times an 
    ' object needs to be regenerated. 
    Public ReadOnly Property RegenerationCount() As Integer 
        Get 
            Return regenCount
        End Get 
    End Property 

    ' Access a data object from the cache. 
    ' If the object was reclaimed for garbage collection, 
    ' create a new data object at that index location.
    Default Public ReadOnly Property Item(ByVal index As Integer) As Data
        Get 
            ' Obtain an instance of a data 
            ' object from the cache of 
            ' of weak reference objects. 
            Dim d As Data = CType(_cache(index).Target, Data)
            If (d Is Nothing) Then 
                ' Object was reclaimed, so generate a new one.
                Console.WriteLine("Regenerate object at {0}: Yes", index.ToString())
                d = New Data(index)
                regenCount += 1
           Else 
                 ' Object was obtained with the weak reference.
                Console.WriteLine("Regenerate object at {0}: No", index.ToString())
            End If 
            Return d
        End Get 

    End Property 
End Class 

' Class that creates byte arrays to simulate data. 
Public Class Data

    Private _data() As Byte

    Private _name As String 

    Public Sub New(ByVal size As Integer)
        MyBase.New()
        _data = New Byte(((size * 1024)) - 1) {}
        _name = size.ToString
    End Sub 

    ' Simple property for 
    ' accessing the object. 
    Public ReadOnly Property Name() As String 
        Get 
            Return _name
        End Get 
    End Property 
End Class 

' Example of the last lines of the output: 
' ... 
' Regenerate object at 36: Yes 
' Regenerate object at 8: Yes 
' Regenerate object at 21: Yes 
' Regenerate object at 4: Yes 
' Regenerate object at 38: No 
' Regenerate object at 7: Yes 
' Regenerate object at 2: Yes 
' Regenerate object at 43: Yes 
' Regenerate object at 38: No 
' Cache size: 50, Regenerated: 94% 
'

System.Object
  System.WeakReference

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

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