.NET并行编程

使用多线程可以利用多核CPU的计算能力，可以提供更好的程序响应能力，但是每个线程都有开销，需要注意控制线程的数量。

1. System.Threading.Thread

使用多线程最直接的是使用System.Threading.Thread。回调函数可以接受一个参数、或者不接受参数，没有返回值。

Thread t = new Thread(Echo);
t.Start("test");

t.Join();

t = new Thread(DoSomeThing);
t.Start();

private static void Echo(object obj)
{
    Console.WriteLine(obj);
    Thread.Sleep(100);
}

private static void DoSomeThing()
{
    Console.WriteLine("DoSomeThing, threadid={0}", Thread.CurrentThread.ManagedThreadId);
    Thread.Sleep(100);
}

2. 线程池

创建和销毁线程需要大量的时间，太多的线程还会消耗大量的内存。使用线程池可以重用线程，方便线程的管理。每个CLR有一个线程池，里面的线程分为Worker和IO线程。

回调函数只能是接受一个参数的函数，没有返回值。

ThreadPool.QueueUserWorkItem(Echo, "test2");

3. 取消

使用CancellationTokenSource和对应的CancellationToken实现协作式取消，CancellationToken是结构类型，包含对CancellationTokenSource的引用。CancellationTokenSource封装了数据，CancellationToken封装了操作。每个线程都有自己的Token，但是访问同一个Source，通过改变Source的状态，可以通知所有监测Token的线程。

CancellationTokenSource cts1 = new CancellationTokenSource();
cts1.Token.Register(
    () =>
        {
            Console.WriteLine("cts1 canceled, threadid={0}", Thread.CurrentThread.ManagedThreadId);
            Thread.Sleep(100);
        });
CancellationTokenSource cts2 = new CancellationTokenSource();
cts2.Token.Register(
    () =>
        {
            Console.WriteLine("cts2 canceled, threadid={0}", Thread.CurrentThread.ManagedThreadId);
            Thread.Sleep(100);
        });
var linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cts1.Token, cts2.Token);
linkedCts.Token.Register(
    () =>
        {
            Console.WriteLine("linkedCts cancled, threadid={0}", Thread.CurrentThread.ManagedThreadId);
            Thread.Sleep(100);
        });

Task.Factory.StartNew(
    () =>
        {
            Thread.Sleep(100);
            if (cts1.Token.IsCancellationRequested)
            {
                Console.WriteLine("Canceled. threadid={0}", Thread.CurrentThread.ManagedThreadId);
            }
        });

cts1.Cancel();

4. Task

线程池的任务没有办法知道状态，也没有返回值。Task建立在线程池基础上，提供了这些功能。

var t = new Task<int>(sum, 20);
t.Start();
Console.WriteLine(t.Result);

Task用法的一个例子。包括TaskFactory，Cancel，ContinueWith

Task parent = new Task(
    () =>
        {
            CancellationTokenSource cts = new CancellationTokenSource();
            TaskFactory<int> tf = new TaskFactory<int>(
                cts.Token,
                TaskCreationOptions.AttachedToParent,
                TaskContinuationOptions.ExecuteSynchronously,
                TaskScheduler.Default);
            var childTasks = new[]
                                 {
                                     tf.StartNew(() => sum(cts.Token, 1000)),
                                     tf.StartNew(() => sum(cts.Token, 2000)),
                                     tf.StartNew(() => sum(cts.Token, int.MaxValue))
                                 };

            for (int i = 0; i < childTasks.Length; i++)
            {
                childTasks[i].ContinueWith(t => cts.Cancel(), TaskContinuationOptions.OnlyOnFaulted);
            }

            tf.ContinueWhenAll(
                childTasks,
                completedTasks =>
                completedTasks.Where(t => !t.IsFaulted && !t.IsCanceled).Max(t => t.Result),
                CancellationToken.None)
                .ContinueWith(
                    t => Console.WriteLine("The max is {0}", t.Result),
                    TaskContinuationOptions.ExecuteSynchronously);
        });

parent.ContinueWith(
    p =>
        {
            StringBuilder sb =
                new StringBuilder(
                    string.Format("Following exception(s) occurred. {0}", Environment.NewLine));
            foreach (var e in p.Exception.Flatten().InnerExceptions)
            {
                sb.AppendLine(string.Format(" {0}: {1}", e.GetType(), e.Message));
            }
            Console.WriteLine(sb.ToString());

        },
    TaskContinuationOptions.OnlyOnFaulted);

parent.Start();

5. APM（Aync Programming Model）