Export (0) Print
Expand All

WeakReference Class

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

System.Object
  System.WeakReference

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

[SerializableAttribute]
[ComVisibleAttribute(true)]
[SecurityPermissionAttribute(SecurityAction.InheritanceDemand, Flags = SecurityPermissionFlag.UnmanagedCode)]
public class WeakReference : ISerializable

The WeakReference type exposes the following members.

  NameDescription
Protected methodWeakReference()Initializes a new instance of the WeakReference class. This constructor overload cannot be implemented in XAML-based applications.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsWeakReference(Object)Initializes a new instance of the WeakReference class, referencing the specified object.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsWeakReference(Object, Boolean)Initializes a new instance of the WeakReference class, referencing the specified object and using the specified resurrection tracking.
Protected methodWeakReference(SerializationInfo, StreamingContext)Initializes a new instance of the WeakReference class, using deserialized data from the specified serialization and stream objects.
Top

  NameDescription
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsIsAliveGets an indication whether the object referenced by the current WeakReference object has been garbage collected.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsTargetGets or sets the object (the target) referenced by the current WeakReference object.
Public propertySupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsTrackResurrectionGets an indication whether the object referenced by the current WeakReference object is tracked after it is finalized.
Top

  NameDescription
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsFinalizeDiscards the reference to the target represented by the current WeakReference object. (Overrides Object.Finalize().)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodGetObjectDataPopulates a SerializationInfo object with all the data needed to serialize the current WeakReference object.
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by the XNA FrameworkSupported by Portable Class LibrarySupported in .NET for Windows Store appsToStringReturns a string that represents the current object. (Inherited from Object.)
Top

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<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.

using System;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        // Create the cache. 
        int cacheSize = 50;
        Random r = new Random();
        Cache c = new Cache(cacheSize);

        string DataName = "";
        GC.Collect(0);

        // Randomly access objects in the cache. 
        for (int i = 0; i < c.Count; i++) {
            int index = r.Next(c.Count);

            // Access the object by getting a property value.
            DataName = c[index].Name;
        }
        // Show results. 
        double regenPercent = c.RegenerationCount/(double)c.Count;
        Console.WriteLine("Cache size: {0}, Regenerated: {1:P2}%", c.Count, regenPercent);
    }
}

public class Cache
{
    // Dictionary to contain the cache. 
    static Dictionary<int, WeakReference> _cache;

    // Track the number of times an object is regenerated. 
    int regenCount = 0;   

    public Cache(int count)
    {
        _cache = new Dictionary<int, WeakReference>();

        // Add objects with a short weak reference to the cache. 
       for (int i = 0; i < count; i++) {
            _cache.Add(i, new WeakReference(new Data(i), false));
        }
    }

    // Number of items in the cache. 
    public int Count
    {
        get {  return _cache.Count; }
    }

    // Number of times an object needs to be regenerated. 
    public int RegenerationCount
    {
        get { return regenCount; }
    }

    // Retrieve a data object from the cache. 
    public Data this[int index]
    {
        get {
            Data d = _cache[index].Target as Data;
            if (d == null) {
                // If the object was reclaimed, generate a new one.
                Console.WriteLine("Regenerate object at {0}: Yes", index);
                d = new Data(index);
                _cache[index].Target = d;
                regenCount++;
            }
            else {
                // Object was obtained with the weak reference.
                Console.WriteLine("Regenerate object at {0}: No", index);
            }

            return d;
       }
    }
}

// This class creates byte arrays to simulate data. 
public class Data
{
    private byte[] _data;
    private string _name;

    public Data(int size)
    {
        _data = new byte[size * 1024];
        _name = size.ToString();
    }

    // Simple property. 
    public string Name
    {
        get { return _name; }
    }
}
// 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%

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
Show:
© 2014 Microsoft