第6章：使用并发集合

6.1、简介

 ConcurrentQueue：队列（Cas原子比较和交换）、ConcurrentStack：堆栈（Cas原子比较和交换）、ConcurrentBag：支持重复的无序集合

　　避免使用Count，复杂度O(N)，IsEmpty属性是O(1)。

 ConncurrentDictionary：字典，读不加锁，写加锁。避免使用Count、IsEmpty、keys、Values、CopyTo以及ToArray。

6.2、ConcurrentDictionary

 写比Dictionary慢、读比Dictionary快，但是只有读取，也不建议用ConcurrentDictionary

        const string Item = "Dictionary Item";
        public static string CuttentItem;

        static void Main(string[] args)
        {
            var concurrentDictionary = new ConcurrentDictionary<int, string>();
            var dictionary = new Dictionary<int, string>();

            var sw = new Stopwatch();
            sw.Start();
            for (int i = 0; i < 1000000; i++)
            {
                lock (dictionary)
                {
                    dictionary[i] = Item;
                }
            }
            sw.Stop();
            Console.WriteLine($"Writing to dictionary with a lock:{sw.Elapsed}");

            sw.Restart();
            for (int i = 0; i < 1000000; i++)
            {
                concurrentDictionary[i] = Item;
            }
            sw.Stop();
            Console.WriteLine($"Writing to concurrent dictionary:{sw.Elapsed}");

            sw.Restart();
            for (int i = 0; i < 1000000; i++)
            {
                lock (dictionary)
                {
                    CuttentItem = dictionary[i];
                }
            }
            sw.Stop();
            Console.WriteLine($"Reading from dictionary with a lock:{sw.Elapsed}");

            sw.Restart();
            for (int i = 0; i < 1000000; i++)
            {
                CuttentItem = concurrentDictionary[i];
            }
            sw.Stop();
            Console.WriteLine($"Reading from concurrent dictionary:{sw.Elapsed}");
            Console.Read();
        }

6.3、使用ConcurrentQueue实现异步处理