使用线程池(4)
实现一个取消选项
简介
当需要中断线程池上的异步操作时,有三种方法实现异步操作的取消:
1. 轮询来检查CancellationToken.IsCancellationRequested属性;
2. 抛出一个OperationCancelledException异;
3. 注册一个回调函数。
以下是代码实践:
using System;
using System.Threading;
namespace 实现一个取消选项
{
internal class Program
{
private static void Main()
{
using (var cts=new CancellationTokenSource())
{
var token = cts.Token;
ThreadPool.QueueUserWorkItem(_ => AsyncOperation1(token));
Thread.Sleep(TimeSpan.FromSeconds(2));
cts.Cancel();
}
using (var cts = new CancellationTokenSource())
{
var token = cts.Token;
ThreadPool.QueueUserWorkItem(_ => AsyncOperation2(token));
Thread.Sleep(TimeSpan.FromSeconds(2));
cts.Cancel();
}
using (var cts = new CancellationTokenSource())
{
var token = cts.Token;
ThreadPool.QueueUserWorkItem(_ => AsyncOperation3(token));
Thread.Sleep(TimeSpan.FromSeconds(2));
cts.Cancel();
}
Console.ReadKey();
}
/*轮询来检查CancellationToken.IsCancellationRequested属性。如果该属性为true,
* 则说明操作需要被取消,我们必须放弃该操作。
*/
private static void AsyncOperation1(CancellationToken token)
{
Console.WriteLine("Starting the first task");
for (var i = 0; i < 5; i++)
{
if (token.IsCancellationRequested)
{//取消操作的过程放在这里
Console.WriteLine("The first task has been canceled.");
return;
}
Thread.Sleep(TimeSpan.FromSeconds(1));
}
Console.WriteLine("The first task has completed succesfully");
}
/*抛出一个OperationCancelledException异常。这允许在操作之外
*控制取消过程,即需要取消操作时,通过操作之外的代码来处理。
*/
private static void AsyncOperation2(CancellationToken token)
{
try
{
Console.WriteLine("Starting the second task");
for (var i = 0; i < 5; i++)
{
token.ThrowIfCancellationRequested();
Thread.Sleep(TimeSpan.FromSeconds(1));
}
Console.WriteLine("The second task has completed successfully");
}
catch (OperationCanceledException)
{//取消操作的过程放在这里
Console.WriteLine("The second task has been canceled.");
}
}
/*注册一个回调函数。当操作被取消时,在线程池将调用改回调函数。
*这允许链式传递一个取消逻辑到另一个异步操作中。
*/
private static void AsyncOperation3(CancellationToken token)
{
var cancellationFlag = false;
token.Register(()=>cancellationFlag=true);//回调函数可以放取消后的操作
Console.WriteLine("Starting the third task");
for (var i = 0; i < 5; i++)
{
if (cancellationFlag)
{//取消操作的过程放在这里
Console.WriteLine("The third task has been canceled.");
return;
}
Thread.Sleep(TimeSpan.FromSeconds(1));
}
Console.WriteLine("The third task has completed successfully");
}
}
}
总结
- ancellationTokenSource和CancellationToken两个新类是在.NET4.0被引入,目前是实现异步操作的取消操作的事实标准;
- 种取消过程根据实际要选取。
备注:学习《Multithreading in C# 5.0 Cookbook》Eugene Agafonov著的总结,以备日后查询。