WaitHandle.WaitOne()中的exitContext参数作用

WaitOne()、WaitAny()、WaitAll()方法中都有一个带有Boolean类型变量的重载方法。msdn对这个变量的介绍是：

true to exit the synchronization domain for the context before the wait (if in a synchronized context), and reacquire it afterward; otherwise, false.

简单的说就是在调用WaitOne方法的时候，是否先退出所在同步域，等方法返回之后再重新获取同步域。
同步域可以理解成只允许单线程执行，不允许并发的一块代码区域。一个线程先进入了同步域，其他线程如果想进入，只能等待这个线程退出同步域。如果WaitOne()执行的时候指定exitContext参数是true，那么WaitOne()等待信号的时候，其他线程可以进入改同步域。
后面的注释有更详细的介绍：

The exitContext parameter has no effect unless the WaitOne method is called from inside a nondefault managed context. This can happen if your thread is inside a call to an instance of a class derived from ContextBoundObject. Even if you are currently executing a method on a class that does not derive from ContextBoundObject, like String, you can be in a nondefault context if a ContextBoundObject is on your stack in the current application domain.

只有在“非默认上下文”中该参数才有效。什么是“非默认上下文”呢，后面的介绍是一个继承了ContextBoundObject的类的实例调用的线程，那么这个线程就在“非默认上下文”中。继承ContextBoundObject类的同时还要使用SynchronizationAttribute属性修饰。

class Program
    {
        static void Main(string[] args)
        {
            BaseClass firstClass = new BaseClass();
            ChildClass secondClass = firstClass.GetChildClass(); 
            Thread firstThread = new Thread(firstClass.method);
            Thread secondThread = new Thread(secondClass.method);
            firstThread.Start();
            Thread.Sleep(500);//先让firstThread执行起来
            secondThread.Start();
            Console.Read();
        }
    }

    [Synchronization(true)]
    class BaseClass : ContextBoundObject//BaseClass继承了ContextBoundObject，并且Synchronization修饰，是一个同步域
    {
        public static ManualResetEvent mre = new ManualResetEvent(false);
        public static Stopwatch stopwatch = new Stopwatch();
        public virtual void method()
        {
            stopwatch.Start();
            Console.WriteLine("BaseClass method begin");
            mre.WaitOne(3000,true);//如果为true,先退出同步域，此时ChildClass可以执行了；如果为false，先不退出，只有BaseClass执行完，ChildClass才能执行
            Console.WriteLine("BaseClass method end after {0} ms",stopwatch.ElapsedMilliseconds);
            stopwatch.Stop();
        }

        public ChildClass GetChildClass()
        {
            return new ChildClass();
        }
    }
    class ChildClass : BaseClass//继承了BaseClass，ChildClass和BaseClass属于同一个同步域
    {
        public override void method()
        {
            Console.WriteLine("ChildClass method entered synchromization domain after {0} ms", stopwatch.ElapsedMilliseconds);
        }
    }

其中mre.WaitOne 方法参数为true，输出如下：

BaseClass method begin
ChildClass method entered synchromization domain after 524 ms
BaseClass method end after 3001 ms

mre.WaitOne 方法参数为false，输出如下：

BaseClass method begin
BaseClass method end after 3008 ms
ChildClass method entered synchromization domain after 3008 ms