第25章 线程处理(5)

25.10 前台线程和后台线程

CLR将每个线程要么视为前台线程，要么视为后台线程。一个进程中的所有前台线程停止运行时，CLR强制终止仍在运行的任何后台线程。这些后台线程被直接终止；不会抛出异常。

前台线程应该用于执行确实想完成的任务，应该为非关键的任务使用后台线程。

CLR要提供前台线程和后台线程的概念更好地支持AppDomain。每个AppDomain都可以运行一个单独的应用程序。每个应用程序都有它自己的前台线程。如果一个应用程序退出，造成它的前台线程终止，则CLR仍然需要保持活动并运行，使其他应用程序继续运行。所有应用程序都退出，它们的所有前台线程都终止后，整个进程就可以被销毁了。

C# Codes：

using System;
using System.Threading;

namespace ThreadStudy
{
    class Program
    {
        static void Main(string[] args)
        {
            Thread thread1 = new Thread(new ThreadStart(Print));
            thread1.IsBackground = true;
            thread1.Start();
            Console.WriteLine("Go on execute main thread~");
            Thread.Sleep(10000);
            Console.WriteLine("Main thread has executed completely~");
        }

        static void Print()
        {
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine("Here is running thread1, {0}% of the thread tasks have been completed~",10*i);
                Thread.Sleep(2000);
            }
        }
    }
}

执行输出：

Here is running thread1, 0% of the thread tasks have been completed~
Go on execute main thread~
Here is running thread1, 10% of the thread tasks have been completed~
Here is running thread1, 20% of the thread tasks have been completed~
Here is running thread1, 30% of the thread tasks have been completed~
Here is running thread1, 40% of the thread tasks have been completed~
Main thread has executed completely~
Here is running thread1, 50% of the thread tasks have been completed~
Press any key to continue . . .

在一个线程的生存期中，任何时候都可以从前台变成后台，或者从后台变成前台。应用程序的主程序以及通过构造一个Thread对象来显式创建的任何线程都默认为前台线程。另一方面，线程池线程默认为后台线程。此外，由进入托管执行环境的本地(native)代码创建的任何线程都被标记为后台线程。