而线程的适用范围则是那种需要长时间 CPU 运算的场合，例如耗时较长的图形处理和算法执行。但是往往由于使用线程编程的简单和符合习惯，所以很多朋友往往会使用线程来执行耗时较长的 I/O 操作。这样在只有少数几个并发操作的时候还无伤大雅，如果需要处理大量的并发操作时就不合适了。
实例研究
说了那么理论上的东西，可能有些兄弟早就不耐烦了，现在我们来研究几个实际的异步操作例子吧。
实例1：由 delegate 产生的异步方法到底是怎么回事？
大家可能都知道，使用 delegate 可以“自动”使一个方法可以进行异步的调用。从直觉上来说，我觉得是由编译器或者 CLR 使用了另外的线程来执行目标方法。到底是不是这样呢？让我们来用一段代码证明一下吧。
using System;
using System.Threading;
namespace AsyncDelegateDemo
{
delegate void AsyncFoo(int i);
class Program
{
///<summary>
/// 输出当前线程的信息
///</summary>
///<param name="name">方法名称</param>
static void PrintCurrThreadInfo(string name)
{
Console.WriteLine("Thread Id of " + name + " is: " + Thread.CurrentThread.ManagedThreadId + ", current thread is "
+ (Thread.CurrentThread.IsThreadPoolThread ? "" : "not ")
+ "thread pool thread.");
}
///<summary>
/// 测试方法，Sleep一定时间
///</summary>
///<param name="i">Sleep的时间</param>
static void Foo(int i)
{
PrintCurrThreadInfo("Foo()");
Thread.Sleep(i);
}
///<summary>
/// 投递一个异步调用
///</summary>
static void PostAsync()
{
AsyncFoo caller = new AsyncFoo(Foo);
caller.BeginInvoke(1000, new AsyncCallback(FooCallBack), caller);
}
static void Main(string[] args)
{
PrintCurrThreadInfo("Main()");
for(int i = 0; i < 10 ; i++)
{
PostAsync();
}
Console.ReadLine();
}
static void FooCallBack(IAsyncResult ar)
{
PrintCurrThreadInfo("FooCallBack()");
AsyncFoo caller = (AsyncFoo) ar.AsyncState;
caller.EndInvoke(ar);
}
}
}
这段代码代码的输出如下：
Thread Id of Main() is: 1, current thread is not thread pool thread.
Thread Id of Foo() is: 3, current thread is thread pool thread.
Thread Id of FooCallBack() is: 3, current thread is thread pool thread.
Thread Id of Foo() is: 3, current thread is thread pool thread.
Thread Id of Foo() is: 4, current thread is thread pool thread.
Thread Id of Foo() is: 5, current thread is thread pool thread.
Thread Id of FooCallBack() is: 3, current thread is thread pool thread.
Thread Id of Foo() is: 3, current thread is thread pool thread.
Thread Id of FooCallBack() is: 4, current thread is thread pool thread.
Thread Id of Foo() is: 4, current thread is thread pool thread.
Thread Id of Foo() is: 6, current thread is thread pool thread.
Thread Id of FooCallBack() is: 5, current thread is thread pool thread.
Thread Id of Foo() is: 5, current thread is thread pool thread.
Thread Id of Foo() is: 7, current thread is thread pool thread.
Thread Id of FooCallBack() is: 3, current thread is thread pool thread.
Thread Id of Foo() is: 3, current thread is thread pool thread.
Thread Id of FooCallBack() is: 4, current thread is thread pool thread.
Thread Id of FooCallBack() is: 6, current thread is thread pool thread.
Thread Id of FooCallBack() is: 5, current thread is thread pool thread.
Thread Id of FooCallBack() is: 7, current thread is thread pool thread.
Thread Id of FooCallBack() is: 3, current thread is thread pool thread.
