c# - 异步/等待异常处理

Question

我的问题基于文章Best Practices in Asynchronous Programming

所以我有这个代码

    async static void AsyncVersion()
    {
        Stopwatch sw = Stopwatch.StartNew();
        string url1 = "http://rsdn.ru";
        string url2 = "http://gotdotnet.ru";
        string url3 = "http://blogs.msdn.com";

        var webRequest1 = WebRequest.Create(url1);
        Console.WriteLine("Before webRequest1.GetResponseAsync(). Thread Id: {0}",
            Thread.CurrentThread.ManagedThreadId);
        ...

所以这段代码会抛出异常，例如当我没有inet连接时。因此，当我遵循msdn的指导并将方法的签名更改为Task（而不是void）时，但是在此之后，它吞噬了所有异常，但是，它们不是由外部 catch块处理的。

整个代码如下

using System;
using System.Diagnostics;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication8
{
    class Program
    {
        async static Task AsyncVersion()
        {
            Stopwatch sw = Stopwatch.StartNew();
            string url1 = "http://rsdn.ru";
            string url2 = "http://gotdotnet.ru";
            string url3 = "http://blogs.msdn.com";

            var webRequest1 = WebRequest.Create(url1);
            Console.WriteLine("Before webRequest1.GetResponseAsync(). Thread Id: {0}",
                Thread.CurrentThread.ManagedThreadId);

            var webResponse1 = await webRequest1.GetResponseAsync();
            Console.WriteLine("{0} : {1}, elapsed {2}ms. Thread Id: {3}", url1,
                webResponse1.ContentLength, sw.ElapsedMilliseconds,
                Thread.CurrentThread.ManagedThreadId);

            var webRequest2 = WebRequest.Create(url2);
            Console.WriteLine("Before webRequest2.GetResponseAsync(). Thread Id: {0}",
                Thread.CurrentThread.ManagedThreadId);

            var webResponse2 = await webRequest2.GetResponseAsync();
            Console.WriteLine("{0} : {1}, elapsed {2}ms. Thread Id: {3}", url2,
                webResponse2.ContentLength, sw.ElapsedMilliseconds,
                Thread.CurrentThread.ManagedThreadId);

            var webRequest3 = WebRequest.Create(url3);
            Console.WriteLine("Before webRequest3.GetResponseAsync(). Thread Id: {0}",
                Thread.CurrentThread.ManagedThreadId);
            var webResponse3 = await webRequest3.GetResponseAsync();
            Console.WriteLine("{0} : {1}, elapsed {2}ms. Thread Id: {3}", url3,
                webResponse3.ContentLength, sw.ElapsedMilliseconds,
                Thread.CurrentThread.ManagedThreadId);
        }
        static void Main(string[] args)
        {

            try
            {
                Console.WriteLine("Main thread id: {0}", Thread.CurrentThread.ManagedThreadId);
                Task task = new Task(() => AsyncVersion());
                task.Start();
                Console.WriteLine("Right after AsyncVersion() method call");
                //Ожидаем завершения асинхронной операции
                task.Wait();
                Console.WriteLine("Asyncronous task finished!");

            }
            catch (Exception e)
            {
                //Все исключения в TPL пробрасываются обернутые в AggregateException
                Console.WriteLine("Exceptopn: {0}", e.Message);
            }
            Console.ReadLine();
        }
    }
}

旧版：

using System;
using System.Diagnostics;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication8
{
    class Program
    {
        async static void AsyncVersion()
        {
            Stopwatch sw = Stopwatch.StartNew();
            string url1 = "http://rsdn.ru";
            string url2 = "http://gotdotnet.ru";
            string url3 = "http://blogs.msdn.com";

            var webRequest1 = WebRequest.Create(url1);
            Console.WriteLine("Before webRequest1.GetResponseAsync(). Thread Id: {0}",
                Thread.CurrentThread.ManagedThreadId);

            var webResponse1 = await webRequest1.GetResponseAsync();
            Console.WriteLine("{0} : {1}, elapsed {2}ms. Thread Id: {3}", url1,
                webResponse1.ContentLength, sw.ElapsedMilliseconds,
                Thread.CurrentThread.ManagedThreadId);

            var webRequest2 = WebRequest.Create(url2);
            Console.WriteLine("Before webRequest2.GetResponseAsync(). Thread Id: {0}",
                Thread.CurrentThread.ManagedThreadId);

            var webResponse2 = await webRequest2.GetResponseAsync();
            Console.WriteLine("{0} : {1}, elapsed {2}ms. Thread Id: {3}", url2,
                webResponse2.ContentLength, sw.ElapsedMilliseconds,
                Thread.CurrentThread.ManagedThreadId);

            var webRequest3 = WebRequest.Create(url3);
            Console.WriteLine("Before webRequest3.GetResponseAsync(). Thread Id: {0}",
                Thread.CurrentThread.ManagedThreadId);
            var webResponse3 = await webRequest3.GetResponseAsync();
            Console.WriteLine("{0} : {1}, elapsed {2}ms. Thread Id: {3}", url3,
                webResponse3.ContentLength, sw.ElapsedMilliseconds,
                Thread.CurrentThread.ManagedThreadId);
        }
        static void Main(string[] args)
        {

            try
            {
                Console.WriteLine("Main thread id: {0}", Thread.CurrentThread.ManagedThreadId);
                Task task = new Task(AsyncVersion);
                task.Start();
                Console.WriteLine("Right after AsyncVersion() method call");
                //Ожидаем завершения асинхронной операции
                task.Wait();
                Console.WriteLine("Asyncronous task finished!");

            }
            catch (System.AggregateException e)
            {
                //Все исключения в TPL пробрасываются обернутые в AggregateException
                Console.WriteLine("AggregateException: {0}", e.InnerException.Message);
            }
            Console.ReadLine();
        }
    }
}

Answer 1

new Task(() => AsyncVersion())

这是有问题的部分。 Task构造函数不理解 async，因此它将忽略从 Task返回的 AsyncVersion。 （您必须使用lambda而不是直接编写 AsyncVersion的事实应该已经使您警觉到正在发生奇怪的事情。）

您有几种解决方法（最后选择最好）：


也将 Task<Task>和 Wait()用作内部 Task：

Task<Task> task = new Task<Task>(AsyncVersion);
task.Start();
task.Result.Wait();

使用 Unwrap()将 Task<Task>更改为代表两个 Task的 Task：

Task<Task> task = new Task<Task>(AsyncVersion);
task.Start();
task.Unwrap().Wait();

使用 Task.Run()，它确实了解 async：

Task task = Task.Run(() => AsyncVersion());
task.Wait();

根本不启动 Task，只需调用 async方法：

Task task = AsyncVersion();
task.Wait();