C# 线程安全集合

转载

对于并行任务，与其相关紧密的就是对一些共享资源，数据结构的并行访问。经常要做的就是对一些队列进行加锁-解锁，然后执行类似插入，删除等等互斥操作。 .NetFramework 4.0 中提供了一些封装好的支持并行操作数据容器，可以减少并行编程的复杂程度。

基本信息

.NetFramework中并行集合的名字空间： System.Collections.Concurrent

并行容器：

•ConcurrentQueue
•ConcurrentStack
•ConcurrentBag ： 一个无序的数据结构集，当不需要考虑顺序时非常有用。
•BlockingCollection ： 与经典的阻塞队列数据结构类似
•ConcurrentDictionary

这些集合在某种程度上使用了无锁技术(CAS Compare-and-Swap和内存屏障 Memory Barrier)，与加互斥锁相比获得了性能的提升。但在串行程序中，最好不用这些集合，它们必然会影响性能。

关于CAS: 

•http://www.tuicool.com/articles/zuui6z
•http://www.360doc.com/content/11/0914/16/7656248_148221200.shtml

关于内存屏障

•http://en.wikipedia.org/wiki/Memory_barrier

用法与示例

ConcurrentQueue

其完全无锁，但当CAS面临资源竞争失败时可能会陷入自旋并重试操作。

•Enqueue：在队尾插入元素
•TryDequeue：尝试删除队头元素，并通过out参数返回
•TryPeek：尝试将对头元素通过out参数返回，但不删除该元素。

程序示例：

using System;
using System.Text;

using System.Threading.Tasks;
using System.Collections.Concurrent;

namespace Sample4_1_concurrent_queue
{
    class Program
    {
        internal static ConcurrentQueue<int> _TestQueue;

        class ThreadWork1  // producer
        {
            public ThreadWork1()
            { }

            public void run()
            {
                System.Console.WriteLine("ThreadWork1 run { ");
                for (int i = ; i < ; i++)
                {
                    System.Console.WriteLine("ThreadWork1 producer: " + i);
                    _TestQueue.Enqueue(i);
                }
                System.Console.WriteLine("ThreadWork1 run } ");
            }
        }

        class ThreadWork2  // consumer
        {
            public ThreadWork2()
            { }

            public void run()
            {
                int i = ;
                bool IsDequeuue = false;
                System.Console.WriteLine("ThreadWork2 run { ");
                for (; ; )
                {
                    IsDequeuue = _TestQueue.TryDequeue(out i);
                    if (IsDequeuue)
                        System.Console.WriteLine("ThreadWork2 consumer: " + i * i + "   =====");

                    if (i == )
                        break;
                }
                System.Console.WriteLine("ThreadWork2 run } ");
            }
        }

        static void StartT1()
        {
            ThreadWork1 work1 = new ThreadWork1();
            work1.run();
        }

        static void StartT2()
        {
            ThreadWork2 work2 = new ThreadWork2();
            work2.run();
        }
        static void Main(string[] args)
        {
            Task t1 = new Task(() => StartT1());
            Task t2 = new Task(() => StartT2());

            _TestQueue = new ConcurrentQueue<int>();

            Console.WriteLine("Sample 3-1 Main {");

            Console.WriteLine("Main t1 t2 started {");
            t1.Start();
            t2.Start();
            Console.WriteLine("Main t1 t2 started }");

            Console.WriteLine("Main wait t1 t2 end {");
            Task.WaitAll(t1, t2);
            Console.WriteLine("Main wait t1 t2 end }");

            Console.WriteLine("Sample 3-1 Main }");

            Console.ReadKey();
        }
    }
}

ConcurrentStack

其完全无锁，但当CAS面临资源竞争失败时可能会陷入自旋并重试操作。

•Push：向栈顶插入元素
•TryPop：从栈顶弹出元素，并且通过out 参数返回
•TryPeek：返回栈顶元素，但不弹出。

程序示例：

using System;
using System.Text;

using System.Threading.Tasks;
using System.Collections.Concurrent;

namespace Sample4_2_concurrent_stack
{
    class Program
    {
        internal static ConcurrentStack<int> _TestStack;

        class ThreadWork1  // producer
        {
            public ThreadWork1()
            { }

            public void run()
            {
                System.Console.WriteLine("ThreadWork1 run { ");
                for (int i = ; i < ; i++)
                {
                    System.Console.WriteLine("ThreadWork1 producer: " + i);
                    _TestStack.Push(i);
                }
                System.Console.WriteLine("ThreadWork1 run } ");
            }
        }

        class ThreadWork2  // consumer
        {
            public ThreadWork2()
            { }

            public void run()
            {
                int i = ;
                bool IsDequeuue = false;
                System.Console.WriteLine("ThreadWork2 run { ");
                for (; ; )
                {
                    IsDequeuue = _TestStack.TryPop(out i);
                    if (IsDequeuue)
                        System.Console.WriteLine("ThreadWork2 consumer: " + i * i + "   =====" + i);

                    if (i == )
                        break;
                }
                System.Console.WriteLine("ThreadWork2 run } ");
            }
        }

        static void StartT1()
        {
            ThreadWork1 work1 = new ThreadWork1();
            work1.run();
        }

        static void StartT2()
        {
            ThreadWork2 work2 = new ThreadWork2();
            work2.run();
        }
        static void Main(string[] args)
        {
            Task t1 = new Task(() => StartT1());
            Task t2 = new Task(() => StartT2());

            _TestStack = new ConcurrentStack<int>();

            Console.WriteLine("Sample 4-1 Main {");

            Console.WriteLine("Main t1 t2 started {");
            t1.Start();
            t2.Start();
            Console.WriteLine("Main t1 t2 started }");

            Console.WriteLine("Main wait t1 t2 end {");
            Task.WaitAll(t1, t2);
            Console.WriteLine("Main wait t1 t2 end }");

            Console.WriteLine("Sample 4-1 Main }");

            Console.ReadKey();
        }
    }
}

测试中一个有趣的现象：

虽然生产者已经在栈中插入值已经到了25，但消费者第一个出栈的居然是4，而不是25。很像是出错了。但仔细想想入栈，出栈和打印语句是两个部分，而且并不是原子操作，出现这种现象应该也算正常。

Sample - Main {
Main t1 t2 started {
Main t1 t2 started }
Main wait t1 t2 end {
ThreadWork1 run {
ThreadWork1 producer:
 ThreadWork2 run {
ThreadWork1 producer:
ThreadWork1 producer:
 ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
 ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
 ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork1 producer:
ThreadWork2 consumer:    =====
ThreadWork2 consumer:    =====
ThreadWork2 consumer:    =====
ThreadWork2 consumer:    =====
ThreadWork1 producer:
ThreadWork1 producer:
 ThreadWork1 producer: 

ConcurrentBag

一个无序的集合，程序可以向其中插入元素，或删除元素。

在同一个线程中向集合插入，删除元素的效率很高。

• Add：向集合中插入元素
• TryTake：从集合中取出元素并删除
• TryPeek：从集合中取出元素，但不删除该元素。

程序示例：

using System;
using System.Text;

using System.Threading.Tasks;
using System.Collections.Concurrent;

namespace Sample4_3_concurrent_bag
{
    class Program
    {
        internal static ConcurrentBag<int> _TestBag;

        class ThreadWork1  // producer
        {
            public ThreadWork1()
            { }

            public void run()
            {
                System.Console.WriteLine("ThreadWork1 run { ");
                for (int i = ; i < ; i++)
                {
                    System.Console.WriteLine("ThreadWork1 producer: " + i);
                    _TestBag.Add(i);
                }
                System.Console.WriteLine("ThreadWork1 run } ");
            }
        }

        class ThreadWork2  // consumer
        {
            public ThreadWork2()
            { }

            public void run()
            {
                int i = ;
                int nCnt = ;
                bool IsDequeuue = false;
                System.Console.WriteLine("ThreadWork2 run { ");
                for (;;)
                {
                    IsDequeuue = _TestBag.TryTake(out i);
                    if (IsDequeuue)
                    {
                        System.Console.WriteLine("ThreadWork2 consumer: " + i * i + "   =====" + i);
                        nCnt++;
                    }

                    if (nCnt == )
                        break;
                }
                System.Console.WriteLine("ThreadWork2 run } ");
            }
        }

        static void StartT1()
        {
            ThreadWork1 work1 = new ThreadWork1();
            work1.run();
        }

        static void StartT2()
        {
            ThreadWork2 work2 = new ThreadWork2();
            work2.run();
        }
        static void Main(string[] args)
        {
            Task t1 = new Task(() => StartT1());
            Task t2 = new Task(() => StartT2());

            _TestBag = new ConcurrentBag<int>();

            Console.WriteLine("Sample 4-3 Main {");

            Console.WriteLine("Main t1 t2 started {");
            t1.Start();
            t2.Start();
            Console.WriteLine("Main t1 t2 started }");

            Console.WriteLine("Main wait t1 t2 end {");
            Task.WaitAll(t1, t2);
            Console.WriteLine("Main wait t1 t2 end }");

            Console.WriteLine("Sample 4-3 Main }");

            Console.ReadKey();
        }
    }
}

BlockingCollection

一个支持界限和阻塞的容器

•Add ：向容器中插入元素
•TryTake：从容器中取出元素并删除
•TryPeek：从容器中取出元素，但不删除。
•CompleteAdding：告诉容器，添加元素完成。此时如果还想继续添加会发生异常。
•IsCompleted：告诉消费线程，生产者线程还在继续运行中，任务还未完成。

示例程序：

程序中，消费者线程完全使用  while (!_TestBCollection.IsCompleted) 作为退出运行的判断条件。

在Worker1中，有两条语句被注释掉了，当i 为50时设置CompleteAdding，但当继续向其中插入元素时，系统抛出异常，提示无法再继续插入。

using System;
using System.Text;

using System.Threading.Tasks;
using System.Collections.Concurrent;

namespace Sample4_4_concurrent_bag
{
    class Program
    {
        internal static BlockingCollection<int> _TestBCollection;

        class ThreadWork1  // producer
        {
            public ThreadWork1()
            { }

            public void run()
            {
                System.Console.WriteLine("ThreadWork1 run { ");
                for (int i = ; i < ; i++)
                {
                    System.Console.WriteLine("ThreadWork1 producer: " + i);
                    _TestBCollection.Add(i);
                    //if (i == 50)
                    //    _TestBCollection.CompleteAdding();
                }
                _TestBCollection.CompleteAdding();

                System.Console.WriteLine("ThreadWork1 run } ");
            }
        }

        class ThreadWork2  // consumer
        {
            public ThreadWork2()
            { }

            public void run()
            {
                int i = ;
                int nCnt = ;
                bool IsDequeuue = false;
                System.Console.WriteLine("ThreadWork2 run { ");
                while (!_TestBCollection.IsCompleted)
                {
                    IsDequeuue = _TestBCollection.TryTake(out i);
                    if (IsDequeuue)
                    {
                        System.Console.WriteLine("ThreadWork2 consumer: " + i * i + "   =====" + i);
                        nCnt++;
                    }
                }
                System.Console.WriteLine("ThreadWork2 run } ");
            }
        }

        static void StartT1()
        {
            ThreadWork1 work1 = new ThreadWork1();
            work1.run();
        }

        static void StartT2()
        {
            ThreadWork2 work2 = new ThreadWork2();
            work2.run();
        }
        static void Main(string[] args)
        {
            Task t1 = new Task(() => StartT1());
            Task t2 = new Task(() => StartT2());

            _TestBCollection = new BlockingCollection<int>();

            Console.WriteLine("Sample 4-4 Main {");

            Console.WriteLine("Main t1 t2 started {");
            t1.Start();
            t2.Start();
            Console.WriteLine("Main t1 t2 started }");

            Console.WriteLine("Main wait t1 t2 end {");
            Task.WaitAll(t1, t2);
            Console.WriteLine("Main wait t1 t2 end }");

            Console.WriteLine("Sample 4-4 Main }");

            Console.ReadKey();
        }
    }
}

当然可以尝试在Work1中注释掉 CompleteAdding 语句，此时Work2陷入循环无法退出。

ConcurrentDictionary

对于读操作是完全无锁的，当很多线程要修改数据时，它会使用细粒度的锁。

•AddOrUpdate：如果键不存在，方法会在容器中添加新的键和值，如果存在，则更新现有的键和值。
•GetOrAdd：如果键不存在，方法会向容器中添加新的键和值，如果存在则返回现有的值，并不添加新值。
•TryAdd：尝试在容器中添加新的键和值。
•TryGetValue：尝试根据指定的键获得值。
•TryRemove：尝试删除指定的键。
•TryUpdate：有条件的更新当前键所对应的值。
•GetEnumerator：返回一个能够遍历整个容器的枚举器。

程序示例：

using System;
using System.Text;

using System.Threading.Tasks;
using System.Collections.Concurrent;

namespace Sample4_5_concurrent_dictionary
{
    class Program
    {
        internal static ConcurrentDictionary<int, int> _TestDictionary;

        class ThreadWork1  // producer
        {
            public ThreadWork1()
            { }

            public void run()
            {
                System.Console.WriteLine("ThreadWork1 run { ");
                for (int i = ; i < ; i++)
                {
                    System.Console.WriteLine("ThreadWork1 producer: " + i);
                    _TestDictionary.TryAdd(i, i);
                }

                System.Console.WriteLine("ThreadWork1 run } ");
            }
        }

        class ThreadWork2  // consumer
        {
            public ThreadWork2()
            { }

            public void run()
            {
                int i = , nCnt = ;
                int nValue = ;
                bool IsOk = false;
                System.Console.WriteLine("ThreadWork2 run { ");
                while (nCnt < )
                {
                    IsOk = _TestDictionary.TryGetValue(i, out nValue);
                    if (IsOk)
                    {
                        System.Console.WriteLine("ThreadWork2 consumer: " + i * i + "   =====" + i);
                        nValue = nValue * nValue;
                        _TestDictionary.AddOrUpdate(i, nValue, (key, value) => { return value = nValue; });
                        nCnt++;
                        i++;
                    }
                }
                System.Console.WriteLine("ThreadWork2 run } ");
            }
        }

        static void StartT1()
        {
            ThreadWork1 work1 = new ThreadWork1();
            work1.run();
        }

        static void StartT2()
        {
            ThreadWork2 work2 = new ThreadWork2();
            work2.run();
        }
        static void Main(string[] args)
        {
            Task t1 = new Task(() => StartT1());
            Task t2 = new Task(() => StartT2());
            bool bIsNext = true;
            int  nValue = ;

            _TestDictionary = new ConcurrentDictionary<int, int>();

            Console.WriteLine("Sample 4-5 Main {");

            Console.WriteLine("Main t1 t2 started {");
            t1.Start();
            t2.Start();
            Console.WriteLine("Main t1 t2 started }");

            Console.WriteLine("Main wait t1 t2 end {");
            Task.WaitAll(t1, t2);
            Console.WriteLine("Main wait t1 t2 end }");

            foreach (var pair in _TestDictionary)
            {
                Console.WriteLine(pair.Key + " : " + pair.Value);
            }

            System.Collections.Generic.IEnumerator<System.Collections.Generic.KeyValuePair<int, int>>
                enumer = _TestDictionary.GetEnumerator();

            while (bIsNext)
            {
                bIsNext = enumer.MoveNext();
                Console.WriteLine("Key: " + enumer.Current.Key +
                                  "  Value: " + enumer.Current.Value);

                _TestDictionary.TryRemove(enumer.Current.Key, out nValue);
            }

            Console.WriteLine("\n\nDictionary Count: " + _TestDictionary.Count);

            Console.WriteLine("Sample 4-5 Main }");

            Console.ReadKey();
        }
    }
}