volatile（C# 参考）

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volatile 关键字指示一个字段可以由多个同时执行的线程修改。声明为 volatile 的字段不受编译器优化（假定由单个线程访问）的限制。这样可以确保该字段在任何时间呈现的都是最新的值。

volatile 修饰符通常用于由多个线程访问但不使用 lock 语句对访问进行序列化的字段。

volatile 关键字可应用于以下类型的字段：

引用类型。
指针类型（在不安全的上下文中）。请注意，虽然指针本身可以是可变的，但是它指向的对象不能是可变的。换句话说，您无法声明“指向可变对象的指针”。
类型，如 sbyte、byte、short、ushort、int、uint、char、float 和 bool。
具有以下基类型之一的枚举类型：byte、sbyte、short、ushort、int 或 uint。
已知为引用类型的泛型类型参数。
IntPtr 和 UIntPtr。

可变关键字仅可应用于类或结构字段。不能将局部变量声明为 volatile。

下面的示例说明如何将公共字段变量声明为 volatile。

    class VolatileTest
    {
        public volatile int i;

        public void Test(int _i)
        {
            i = _i;
        }
    }

示例

下面的示例演示如何创建辅助线程，并用它与主线程并行执行处理。有关多线程处理的背景信息，请参见Threading和线程。

using System;
using System.Threading;

public class Worker
{
    // This method is called when the thread is started.
    public void DoWork()
    {
        while (!_shouldStop)
        {
            Console.WriteLine("Worker thread: working...");
        }
        Console.WriteLine("Worker thread: terminating gracefully.");
    }
    public void RequestStop()
    {
        _shouldStop = true;
    }
    // Keyword volatile is used as a hint to the compiler that this data
    // member is accessed by multiple threads.
    private volatile bool _shouldStop;
}

public class WorkerThreadExample
{
    static void Main()
    {
        // Create the worker thread object. This does not start the thread.
        Worker workerObject = new Worker();
        Thread workerThread = new Thread(workerObject.DoWork);

        // Start the worker thread.
        workerThread.Start();
        Console.WriteLine("Main thread: starting worker thread...");

        // Loop until the worker thread activates.
        while (!workerThread.IsAlive) ;

        // Put the main thread to sleep for 1 millisecond to
        // allow the worker thread to do some work.
        Thread.Sleep(1);

        // Request that the worker thread stop itself.
        workerObject.RequestStop();

        // Use the Thread.Join method to block the current thread 
        // until the object's thread terminates.
        workerThread.Join();
        Console.WriteLine("Main thread: worker thread has terminated.");
    }
    // Sample output:
    // Main thread: starting worker thread...
    // Worker thread: working...
    // Worker thread: working...
    // Worker thread: working...
    // Worker thread: working...
    // Worker thread: working...
    // Worker thread: working...
    // Worker thread: terminating gracefully.
    // Main thread: worker thread has terminated.
}