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

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 = "";

        // 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 * 100 / c.Count;
        Console.WriteLine("Cache size: {0}, Regenerated: {1}%", c.Count.ToString(), regenPercent.ToString());

    }
}


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 data 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));
        }

    }

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

    // Returns the number of times an  
    // object had to be regenerated. 
    public int RegenerationCount
    {
        get
        {
            return regenCount;
        }
    }

    // Accesses a data object from the cache. 
    // If the object was reclaimed for garbage collection, 
    // create a new data object at that index location. 
    public Data this[int index]
    {
        get
        {
            // Obtain an instance of a data 
            // object from the cache of 
            // of weak reference objects.
            Data d = _cache[index].Target as Data;
            if (d == null)
            {
                // Object was reclaimed, so generate a new one.
                Console.WriteLine("Regenerate object at {0}: Yes", index.ToString());
                d = new Data(index);
                regenCount++;
            }
            else
            {
                // Object was obtained with the weak reference.
                Console.WriteLine("Regenerate object at {0}: No", index.ToString());
            }

            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% 
//
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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