并发编程技术之一 ———— C#多线程基础

1. C#多线程技术简介

线程是一个独立的运行单元，每个进程内部有多个线程，每个线程可以各自同时执行指令。每个线程有自己独立的栈，但是与进程内的其他线程共享内存。对某些程序来说，其中有一个线程是特殊的，例如：Console的main线程和窗体程序的UI线程。

每个.NET程序都有一个线程池，线程池维护着一定数量的工作线程，这些线程等待着执行分配下来的任务，线程池可以随时监测线程的数量。配置线程池的参数很多。但是我都建议大家使用默认值，这些值都是经过微软调试好的，可以满足大部分应用。

线程是低级别的抽象，线程池虽然高级一点，但同样很低，而现在C#给我们提供了很多高级的并发编程技术工具，所以原则上我们不建议直接操作Thread对象。但是为了让大家很好的理解C#多线程的来龙去脉，这里介绍C#最初操作多线程的方法。

2 实战：第一个程序

由于写博客我喜欢用Mac，所以我的例程是用.net core 框架，使用 VS Code 开发，源码发布到github中，链接：spartajet/CSharpLearnBlog

2.1 C# 创建线程

首先，我们要创建一个新建线程中运行的方法，主要是打印数字，如下：

/// <summary>
/// 数数方法
/// </summary>
static void NumberCount(){
for (int i = 0; i < 10; i++)
    {
        Console.WriteLine($"The number is {i}");
    }
}

在 main方法中添加如下代码来开启一个线程

static void Main(string[] args)
{
    Thread workThread=new Thread(NumberCount);
    workThread.Start();
    NumberCount();
}

运行结果，如下：

The number is 0
The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9

可以看出，C#的多线程执行顺序是不确定的。NumberCount方法同时被工作线程和主线程调用，可以看到工作线程和主线程是同步运行的，互不干扰。

Thread的声明还有其他方式，可以使用Lambda风格来定义。方法如下：

var workThread=new Thread(() =>
{
    for (int i = 0; i < 10; i++)
    {
        Console.WriteLine($"The number is {i}");
    }
});

2.2 暂停一个线程

暂停一个线程是让一个线程等待一段时间而不消耗操作系统资源。

将方法NumberCount 修改为：

/// <summary>
/// 数数方法
/// </summary>
static void NumberCountCouldDelay(){
for (int i = 0; i < 10; i++)
    {
        Console.WriteLine($"The number is {i}");
        Thread.Sleep(TimeSpan.FromSeconds(1));
    }
}

运行结果为：

The number is 0
The number is 0
The number is 1
The number is 1
The number is 2
The number is 2
The number is 3
The number is 3
The number is 4
The number is 4
The number is 5
The number is 5
The number is 6
The number is 6
The number is 7
The number is 7
The number is 8
The number is 8
The number is 9
The number is 9

大家可以看到，方法改动只是增加了一行

 Thread.Sleep(TimeSpan.FromSeconds(1));

Thread.Sleep 方法被调用后，线程处于休眠状态，会尽可能的少占用系统资源，起到了暂停线程的效果。

2.3 线程等待

线程等待是指多线程编程中，一个线程等待另一个线程完成后再执行。

代码如下：

static void Main(string[] args)
{
    var workThread=new Thread(NumberCountCoudDelay);
    workThread.Start();
    workThread.Join();
    NumberCount();
}
/// <summary>
/// 数数方法
/// </summary>
static void NumberCountCoudDelay(){
    for (int i = 0; i < 10; i++)
    {
        Console.WriteLine($"Delay thread number is {i}");
        Thread.Sleep(TimeSpan.FromSeconds(1));
    }
}
/// <summary>
/// 数数方法
/// </summary>
static void NumberCount(){
    for (int i = 0; i < 10; i++)
    {
        Console.WriteLine($"Main thread number is {i}");
        Thread.Sleep(TimeSpan.FromSeconds(1));
    }
}

运行结果：

Delay thread number is 0
Delay thread number is 1
Delay thread number is 2
Delay thread number is 3
Delay thread number is 4
Delay thread number is 5
Delay thread number is 6
Delay thread number is 7
Delay thread number is 8
Delay thread number is 9
Main thread number is 0
Main thread number is 1
Main thread number is 2
Main thread number is 3
Main thread number is 4
Main thread number is 5
Main thread number is 6
Main thread number is 7
Main thread number is 8
Main thread number is 9

可以看出来，我们使用了 Join 方法，该方法允许我们等待知道此线程完成，当线程完成后，下面的代码才可以执行。

2.4 结束线程

结束线程是指在某个线程运行中间停止该线程。

代码如下：

static void Main(string[] args)
{
    var workThread=new Thread(NumberCountCoudDelay);
    workThread.Start();
    Thread.Sleep(TimeSpan.FromSeconds(4));
    workThread.Abort();
    Console.WriteLine("Work thread is stopped!!!");
}

结果如下：

Delay thread number is 0
Delay thread number is 1
Delay thread number is 2
Delay thread number is 3

Unhandled Exception: Delay thread number is 4
System.PlatformNotSupportedException: Thread abort is not supported on this platform.
   at System.Threading.Thread.Abort()
   at CSharpAsync.Program.Main(String[] args) in /Users/spartajet/CodeWorkSpace/VSCode/CSharpLearnBlog/CSharpAsync/Program.cs:line 12

为什么会出现这个现象呢，请参照：Methods that throw PlatformNotSupportedException are not documented

可以看出.net core 对 Thread 的支持不够，是因为 Thread 已经完全落伍了，同样在Windows我们也不建议这么做。
方法代码如下：

    public void Abort()
    {
      throw new PlatformNotSupportedException(SR.PlatformNotSupported_ThreadAbort);
    }

    public void Abort(object stateInfo)
    {
      throw new PlatformNotSupportedException(SR.PlatformNotSupported_ThreadAbort);
    }

    public static void ResetAbort()
    {
      throw new PlatformNotSupportedException(SR.PlatformNotSupported_ThreadAbort);
    }

    [Obsolete("Thread.Suspend has been deprecated.  Please use other classes in System.Threading, such as Monitor, Mutex, Event, and Semaphore, to synchronize Threads or protect resources.  http://go.microsoft.com/fwlink/?linkid=14202", false)]
    public void Suspend()
    {
      throw new PlatformNotSupportedException(SR.PlatformNotSupported_ThreadSuspend);
    }

但是在Windows平台这段代码是完全可行的。

2.5 检测线程状态

一个线程可以用 ThreadState 枚举来表示，

下面是ThreadState的源码：

public enum ThreadState
{
  Running = 0,
  StopRequested = 1,
  SuspendRequested = 2,
  Background = 4,
  Unstarted = 8,
  Stopped = 16, // 0x00000010
  WaitSleepJoin = 32, // 0x00000020
  Suspended = 64, // 0x00000040
  AbortRequested = 128, // 0x00000080
  Aborted = 256, // 0x00000100
}

测试代码如下：

static void Main(string[] args)
{
    var workThread=new Thread(NumberCountCoudDelay);
    Console.WriteLine($"work thread state: {workThread.ThreadState}");
    workThread.Start();
    workThread.Join();
    Console.WriteLine($"work thread state: {workThread.ThreadState}");
    Console.WriteLine("Work thread is stopped!!!");
}

结果如下：

work thread state: Unstarted
Delay thread number is 0
work thread state: Running
Delay thread number is 1
work thread state: Running
Delay thread number is 2
work thread state: Running
Delay thread number is 3
work thread state: Running
Delay thread number is 4
work thread state: Running
Delay thread number is 5
work thread state: Running
Delay thread number is 6
work thread state: Running
Delay thread number is 7
work thread state: Running
Delay thread number is 8
work thread state: Running
Delay thread number is 9
work thread state: Running
work thread state: Stopped
Work thread is stopped!!!