【不小心就会犯错】 .NET的ConcurrentDictionary,线程安全集合类

ConcurrentDictionary 是.NET 4.0里面新增的号称线程安全的集合类。

那么自然,我们会预期ConcurrentDictionary 中的代码是线程安全的(至少几个关键方法是线程安全的).

举个例子,使用者可能会预期GetOrAdd中的方法当Key不存在的时候只执行一次Add的委托,第二次调用GetOrAdd就应该直接取回刚才生成的值了.

参考一下以下代码:

        public static  void Test()
        {
            var concurentDictionary = new ConcurrentDictionary<int, int>();

            var w = new ManualResetEvent(false);
            int timedCalled = 0;
            var threads = new List<Thread>();
            for (int i = 0; i < Environment.ProcessorCount; i++)
            {
                threads.Add(new Thread(() =>
                {
                    w.WaitOne();
                    concurentDictionary.GetOrAdd(1, i1 =>
                    {
                        Interlocked.Increment(ref timedCalled);
                        return 1;
                    });
                }));
                threads.Last().Start();
            }

            w.Set();//release all threads to start at the same time         
            Thread.Sleep(100);
            Console.WriteLine(timedCalled);// output is 4, means call initial 4 times
            //Console.WriteLine(concurentDictionary.Keys.Count);
        }

GetOrAdd方法的定义就是按照Key获取一个Value,如果Key不存在，那么调用Func<T> 添加一个键值对.

按照ConcurrentDictionary的定义, 我预期这个Add应该只被调用一次

可是上面那段代码的运行结果表明， Interlocked.Increment(ref timedCalled); 被调用了4次,真是尴尬啊

用于初始化值的委托还真的是可以多次执行的,所以

要么保证委托中的代码重复执行不会有问题
要么使用线程安全的初始化方法,例如Lazy<T>

 public static void Test()
        {
            var concurentDictionary = new ConcurrentDictionary<int, int>();

            var w = new ManualResetEvent(false);
            int timedCalled = 0;
            var threads = new List<Thread>();
            Lazy<int> lazy = new Lazy<int>(() => { Interlocked.Increment(ref timedCalled); return 1; });
            for (int i = 0; i < Environment.ProcessorCount; i++)
            {
                threads.Add(new Thread(() =>
                {
                    w.WaitOne();
                    concurentDictionary.GetOrAdd(1, i1 =>
                    {
                        return lazy.Value;
                    });
                }));
                threads.Last().Start();
            }

            w.Set();//release all threads to start at the same time         
            Thread.Sleep(100);
            Console.WriteLine(timedCalled);// output is 1
        }

附: 注释中也不说一下这个初始化方法会被多次调用,如果不是偶然遇到这个问题,估计永远都不知道

 //
        // Summary:
        //     Adds a key/value pair to the System.Collections.Concurrent.ConcurrentDictionary<TKey,TValue>
        //     if the key does not already exist.
        //
        // Parameters:
        //   key:
        //     The key of the element to add.
        //
        //   valueFactory:
        //     The function used to generate a value for the key
        //
        // Returns:
        //     The value for the key. This will be either the existing value for the key
        //     if the key is already in the dictionary, or the new value for the key as
        //     returned by valueFactory if the key was not in the dictionary.
        //
        // Exceptions:
        //   System.ArgumentNullException:
        //     key is a null reference (Nothing in Visual Basic).-or-valueFactory is a null
        //     reference (Nothing in Visual Basic).
        //
        //   System.OverflowException:
        //     The dictionary contains too many elements.