C# WeakReference 对象在终结器中为 NULL，但仍被强引用

C# WeakReference object is NULL in finalizer although still strongly referenced(C# WeakReference 对象在终结器中为 NULL，但仍被强引用)

问题描述

您好，我这里有代码，但我不明白为什么会遇到断点(请参阅评论).

Hi I have code here where I don't understand why I hit the breakpoint (see comment).

这是我不知道或我不正确理解的 Microsoft 错误吗?

Is this a Microsoft bug of something I don't know or I don't understand properly ?

代码在 Debug 中经过测试，但我认为它不应该改变任何东西.

The code was tested in Debug but I think it should not changes anything.

注意:您可以直接在控制台应用程序中测试代码.

Note: You can test the code directly in a console app.

仅供参考...在 supercat 回答之后，我用建议的解决方案修复了我的代码，它运行良好:-) !!!坏事是静态字典的使用以及随之而来的性能，但它确实有效....几分钟后，我意识到 SuperCat 给了我所有提示，让我做得更好，解决静态字典，我做到了.代码示例是:

JUST FOR INFORMATION... following supercat answer, I fixed my code with proposed solution and it works nicely :-) !!! The bad thing is the usage of a static dict and the performance the goes with it but it works. ... After few minutes, I realized that SuperCat give me all hints to do it better, to workaround the static dictionary and I did it. Code samples are:

1. 有错误的代码
2. 代码已更正，但使用静态 ConditionalWeakTable
3. 包含 SuperCat 技巧的 ConditioalWeakTable 代码(非常感谢他！)

样品...

using System;
using System.Collections.Generic;
using System.Diagnostics;

namespace WeakrefBug
{

// **********************************************************************
class B : IDisposable
{
    public static List<B> AllBs = new List<B>();

    public B()
    {
        AllBs.Add(this);
    }

    private bool disposed = false;
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (!disposed)
        {
            AllBs.Remove(this);
            disposed = true;
        }
    }

    ~B() { Dispose(false); }
}

// **********************************************************************
class A
{
    WeakReference _weakB = new WeakReference(new B());

    ~A()
    {
        B b = _weakB.Target as B;
        if (b == null)
        {
            if (B.AllBs.Count == 1)
            {
                Debugger.Break(); // b Is still referenced but my weak reference can't find it, why ?
            }
        }
        else { b.Dispose(); }
    }
}

// **********************************************************************
class Program
{
    static void Main(string[] args)
    {
        A a = new A();
        a = null;

        GC.Collect(GC.MaxGeneration);
        GC.WaitForPendingFinalizers();
    }
    }

    // **********************************************************************
}

版本更正:

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.CompilerServices;

namespace WeakrefBug // Working fine with ConditionalWeakTable
{
    // **********************************************************************
    class B : IDisposable
    {
        public static List<B> AllBs = new List<B>();

        public B()
        {
            AllBs.Add(this);
        }

        private bool disposed = false;
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected virtual void Dispose(bool disposing)
        {
            if (!disposed)
            {
                AllBs.Remove(this);
                disposed = true;
            }
        }

        ~B() { Dispose(false); }
    }

    // **********************************************************************
    class A
    {
        private static readonly System.Runtime.CompilerServices.ConditionalWeakTable<A, B> WeakBs = new ConditionalWeakTable<A, B>();

        public A()
        {
            WeakBs.Add(this, new B());          
        }

        public B CreateNewB()
        {
            B b = new B();
            WeakBs.Remove(this);
            WeakBs.Add(this, b);
            return b;
        }

        ~A()
        {
            B b;
            WeakBs.TryGetValue(this, out b);

            if (b == null)
            {
                if (B.AllBs.Count == 1)
                {
                    Debugger.Break(); // B Is still referenced but my weak reference can't find it, why ?
                }
            }
            else { b.Dispose(); }
        }
    }

    // **********************************************************************
    class Program
    {
        static void Main(string[] args)
        {
            A a = new A();
            WeakReference weakB = new WeakReference(a.CreateNewB()); // Usually don't need the internal value, but only to ensure proper functionnality
            a = null;

            GC.Collect(GC.MaxGeneration);
            GC.WaitForPendingFinalizers();

            Debug.Assert(!weakB.IsAlive);
        }
    }

    // **********************************************************************
}

包含 SuperCat 技巧的 ConditioalWeakTable 代码(非常感谢他！)

Code with ConditioalWeakTable that include the SuperCat tricks (thanks so much to him !)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.CompilerServices;

namespace WeakrefBug // Working fine with non static ConditionalWeakTable - auto cleanup
{
    // **********************************************************************
    class B : IDisposable
    {
        public static List<B> AllBs = new List<B>();

        public B()
        {
            AllBs.Add(this);
        }

        private bool disposed = false;
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected virtual void Dispose(bool disposing)
        {
            if (!disposed)
            {
                AllBs.Remove(this);
                disposed = true;
            }
        }

        ~B() { Dispose(false); }
    }

    // **********************************************************************
    class A
    {
        private ConditionalWeakTable<object, object> _weakBs = null;

        public A()
        {
        }

        public B CreateNewB()
        {
            B b = new B();
            if (_weakBs == null)
            {
                _weakBs = new ConditionalWeakTable<object, object>();
                _weakBs.Add(b, _weakBs);
            }
            _weakBs.Remove(this);
            _weakBs.Add(this, b);
            return b;
        }

        internal ConditionalWeakTable<object, object> ConditionalWeakTable // TestOnly
        {
            get { return _weakBs; }
        }

        ~A()
        {
            object objB;
            _weakBs.TryGetValue(this, out objB);

            if (objB == null)
            {
                if (B.AllBs.Count == 1)
                {
                    Debugger.Break(); // B Is still referenced but my weak reference can't find it, why ?
                }
            }
            else
            {
                ((B)objB).Dispose();
            }
        }
    }

    // **********************************************************************
    class Program
    {
        static void Main(string[] args)
        {
            A a = new A();
            WeakReference weakB = new WeakReference(a.CreateNewB()); // Usually don't need the internal value, but only to ensure proper functionnality
            WeakReference weakConditionalWeakTable = new WeakReference(a.ConditionalWeakTable);
            a = null;

            GC.Collect(GC.MaxGeneration);
            GC.WaitForPendingFinalizers();

            Debug.Assert(!weakB.IsAlive);
            Debug.Assert(!weakConditionalWeakTable.IsAlive);
        }
    }

    // **********************************************************************

}

CitizenInsane 的以下问题...我不记得我到底为什么要这么做……我找到了我的样本，但当时不确定我的意图.我试图弄清楚并附带了以下代码，我认为它更清楚，但仍然不记得我最初的需求.对不起???

Following question of CitizenInsane... I don't remember exactly why I did what I did... I found my sample but wasn't sure about my intention at that time. I tried to figure it out and came with the following code which I thing is more clear but still don't remember my original need. Sorry ???

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.CompilerServices;

namespace WeakrefBug // Working fine with ConditionalWeakTable
{
    // **********************************************************************
    class B : IDisposable
    {
        public static List<B> AllBs = new List<B>();

        public B()
        {
            AllBs.Add(this);
        }

        private bool disposed = false;
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        protected virtual void Dispose(bool disposing)
        {
            if (!disposed)
            {
                AllBs.Remove(this);
                disposed = true;
            }
        }

        ~B() { Dispose(false); }
    }

    // **********************************************************************
    class A
    {
        private ConditionalWeakTable<object, object> _weakBs = null;
        private WeakReference _weakB = null;

        public A()
        {
            _weakBs = new ConditionalWeakTable<object, object>();
            B b = new B();
            _weakB = new WeakReference(b);
            _weakBs.Add(b, _weakB);
        }

        public B B
        {
            get
            {
                return _weakB.Target as B;
            }
            set { _weakB.Target = value; }
        }

        internal ConditionalWeakTable<object, object> ConditionalWeakTable // TestOnly
        {
            get { return _weakBs; }
        }

        ~A()
        {
            B objB = B;

            if (objB == null)
            {
                if (B.AllBs.Count == 1)
                {
                    Debugger.Break(); // B Is still referenced but my weak reference can't find it, why ?
                }
            }
            else
            {
                ((B)objB).Dispose();
            }
        }
    }

    // **********************************************************************
    class Program
    {
        static void Main(string[] args)
        {
            Test1();
            Test2();
        }

        private static void Test1()
        {
            A a = new A();
            WeakReference weakB = new WeakReference(a.B); // Usually don't need the internal value, but only to ensure proper functionnality
            WeakReference weakConditionalWeakTable = new WeakReference(a.ConditionalWeakTable);

            a = null;

            GC.Collect(GC.MaxGeneration);
            GC.WaitForPendingFinalizers();

            Debug.Assert(B.AllBs.Count == 0);

            GC.Collect(GC.MaxGeneration);
            GC.WaitForPendingFinalizers();

            Debug.Assert(!weakB.IsAlive); // Need  second pass of Collection to be collected
            Debug.Assert(!weakConditionalWeakTable.IsAlive);
        }

        private static void Test2()
        {
            A a = new A();
            WeakReference weakB = new WeakReference(a.B);

            B.AllBs.Clear();
            a.B = null;

            GC.Collect(GC.MaxGeneration);
            GC.WaitForPendingFinalizers();

            Debug.Assert(!weakB.IsAlive); // Need  second pass of Collection to be collected
        }
    }

    // **********************************************************************

}

推荐答案

WeakReference 的一个有时令人讨厌的限制是，如果没有强根，WeakReference 可能会失效WeakReference itself 的引用存在，即使 trackResurrection 构造函数参数为 true 并且 即使 WeakReference 的目标是强根.这种行为源于 WeakReference 具有非托管资源(GC 句柄)，如果 WeakReference 的终结器没有清理 GC 句柄，它会永远不会被清理，会构成内存泄漏.

A sometimes-irksome limitation of WeakReference is that a WeakReference may be invalidated if no strongly-rooted reference exists to the WeakReference itself, and this may occur even if the trackResurrection constructor parameter was true, and even if the target of the WeakReference is strongly rooted. This behavior stems from the fact that a WeakReference has an unmanaged resource (a GC handle) and if the finalizer for the WeakReference didn't clean up the GC handle, it would never get cleaned up and would constitute a memory leak.

如果对象的终结器需要使用 WeakReference 对象，则该对象必须做出一些规定以确保这些对象保持强引用.我不确定完成此操作的最佳模式是什么，但在 .net 4.0 中添加的 ConditionalWeakTable<TKey,TValue> 可能很有用.有点像Dictionary，只不过只要一个表本身被强引用，给定的key被强引用，其对应的值就会被认为是强引用的.请注意，如果 ConditionalWeakTable 包含将 X 链接到 Y 以及将 Y 链接到表的条目，那么只要 X 或 Y 保留，表也将保留.

If it will be necessary for an object's finalizers to make use of WeakReference objects, the object must make some provision to ensure that those objects remain strongly referenced. I'm not sure what the best pattern is to accomplish this, but the ConditionalWeakTable<TKey,TValue> that was added in .net 4.0 may be useful. It's a little bit like Dictionary<TKey,TValue> except that as long as a table itself is strongly referenced and a given key is strongly referenced, its corresponding value will be regarded as strongly referenced. Note that if a ConditionalWeakTable holds an entry linking X to Y, and Y to the table, then as long as X or Y remains, the table will remain as well.

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