WeakReference Class
Represents a weak reference, which references an object while still allowing that object to be reclaimed by garbage collection.
Assembly: mscorlib (in mscorlib.dll)
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% '
- SecurityPermission
for the ability to call unmanaged code. Demand value: InheritanceDemand; Permission value: UnmanagedCode
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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.