C#当中的多线程_任务并行库(下)

4.8 处理任务中的异常

下面这个例子讨论了任务当中抛出异常，以及任务异常的获取

     class Program
     {
         static void Main(string[] args)
         {
             //声明一个任务
             Task<int> task;
             //第一种方式，普通的try...catch捕获异常
             try
             {
                 task = Task.Run(() => TaskMethod("Task 1", ));
                 int result = task.Result;
                 Console.WriteLine("Result: {0}", result);
             }
             catch (Exception ex)
             {
                 //这里捕获的异常是一个被封装的异常，叫做AggregateException
                 Console.WriteLine("Exception caught: {0}", ex);
                 Console.WriteLine("----------------------------------------------");
                 //本例中AggregateException之中只有一个异常，因为只有一个任务抛出了异常
                 Console.WriteLine("InnerException is {0}",ex.InnerException.ToString());
             }
             Console.WriteLine("----------------------------------------------");
             Console.WriteLine();

             //第二种方式是采用GetAwaiter().GetResult()方法来访问任务结果，这种方式可以提取没有封装的异常
             try
             {
                 task = Task.Run(() => TaskMethod("Task 2", ));
                 int result = task.GetAwaiter().GetResult();
                 Console.WriteLine("Result: {0}", result);
             }
             catch (Exception ex)
             {
                 Console.WriteLine("Exception caught: {0}", ex);
             }
             Console.WriteLine("----------------------------------------------");
             Console.WriteLine();

             //第三个demo展示了两个任务抛出异常的情形。
             var t1 = new Task<int>(() => TaskMethod("Task 3", ));
             var t2 = new Task<int>(() => TaskMethod("Task 4", ));
             var complexTask = Task.WhenAll(t1, t2);
             var exceptionHandler = complexTask.ContinueWith(t =>
                     Console.WriteLine("Exception caught: {0}", t.Exception),
                     TaskContinuationOptions.OnlyOnFaulted
                 );
             t1.Start();
             t2.Start();

             Thread.Sleep(TimeSpan.FromSeconds());
         }

         static int TaskMethod(string name, int seconds)
         {
             Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",
                 name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
             Thread.Sleep(TimeSpan.FromSeconds(seconds));
             //抛出一个异常
             throw new Exception("Boom!");
             return  * seconds;
         }
    }

结果如下

第一个demo

第二个demo

第三个demo

4.9 并行运行任务
这个小节主要讲述了Task.WhenAll()和Task.WhenAny()这两个方法，一个是等待所有任务全执行后的操作，一个是等待任何一个任务执行完的操作。

例子：
 class Program
     {
         static void Main(string[] args)
         {
             //创建两个任务firstTask和secondTask
             var firstTask = new Task<int>(() => TaskMethod("First Task", ));
             var secondTask = new Task<int>(() => TaskMethod("Second Task", ));
             //借助Task.WhenAll()方法创建第三个任务，该任务会在前两个任务完成后完成
             var whenAllTask = Task.WhenAll(firstTask, secondTask);
           //whenAllTask任务结束以后会产生一个结果数组，
           //对应的第一个元素是第一个任务结果，第二个元素对应的是第二个任务结果...
           whenAllTask.ContinueWith(t =>
                 Console.WriteLine("The first answer is {0}, the second is {1}", t.Result[], t.Result[]),
                 TaskContinuationOptions.OnlyOnRanToCompletion
                 );

            firstTask.Start();
            secondTask.Start();

            Thread.Sleep(TimeSpan.FromSeconds());

             //这个demo，展示了启动一系列任务运行的过程
             var tasks = new List<Task<int>>();
             for (int i = ; i < ; i++)
             {
                 int counter = i;
                 var task = new Task<int>(() => TaskMethod(string.Format("Task {0}", counter), counter));
                 tasks.Add(task);
                 task.Start();
             }

             while (tasks.Count > )
             {
                 //使用Task.WhenAny()方法等待任务当中的任何一个任务完成
                 var completedTask = Task.WhenAny(tasks).Result;
                 //每当一个任务完成，就把他从任务列表移除，直到任务列表为空
                 tasks.Remove(completedTask);
                 Console.WriteLine("A task has been completed with result {0}.", completedTask.Result);
             }

             Thread.Sleep(TimeSpan.FromSeconds());
         }

         static int TaskMethod(string name, int seconds)
         {
             Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",
                 name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
             Thread.Sleep(TimeSpan.FromSeconds(seconds));
             return  * seconds
         }