Question

假设以下同步代码：

try
{
    Foo();
    Bar();
    Fubar();
    Console.WriteLine("All done");
}
catch(Exception e) // For illustration purposes only. Catch specific exceptions!
{
    Console.WriteLine(e);
}

现在假设所有这些方法都有一个Async对应物，我必须出于某种原因使用它们，所以简单地将整个事物包装在新任务中不是一种选择。
我将如何实现相同的行为？
我对“相同”的意思是：

如果抛出异常，则执行异常处理程序。
如果抛出异常，请停止执行以下方法。

我唯一能想到的是可怕：

var fooTask = FooAsync();
fooTask.ContinueWith(t => HandleError(t.Exception),
                     TaskContinuationOptions.OnlyOnFaulted);
fooTask.ContinueWith(
    t =>
    {
        var barTask = BarAsync();
        barTask.ContinueWith(t => HandleError(t.Exception),
                             TaskContinuationOptions.OnlyOnFaulted);
        barTask.ContinueWith(
            t =>
            {
                var fubarTask = FubarAsync();
                fubarTask.ContinueWith(t => HandleError(t.Exception),
                                       TaskContinuationOptions.OnlyOnFaulted);
                fubarTask.ContinueWith(
                    t => Console.WriteLine("All done"),
                    TaskContinuationOptions.OnlyOnRanToCompletion);
            }, 
            TaskContinuationOptions.OnlyOnRanToCompletion);
    }, 
    TaskContinuationOptions.OnlyOnRanToCompletion);

请注意：

我需要一个适用于.NET 4的解决方案，因此async/await是不可能的。但是，如果它可以与async/await一起使用，请随时展示如何。
我不需要使用TPL。如果使用TPL是不可能的，那么另一种方法就可以了，可能还有Reactive Extensions？

Answer 1

以下是async：

的用法

try
{
    await FooAsync();
    await BarAsync();
    await FubarAsync();
    Console.WriteLine("All done");
}
catch(Exception e) // For illustration purposes only. Catch specific exceptions!
{
    Console.WriteLine(e);
}

如果您安装了（预发布）Microsoft.Bcl.Async package。

，这将适用于.NET 4.0

由于您坚持使用VS2010，因此可以使用Stephen Toub's Then的变体：

public static Task Then(this Task first, Func<Task> next)
{
  var tcs = new TaskCompletionSource<object>();
  first.ContinueWith(_ =>
  {
    if (first.IsFaulted) tcs.TrySetException(first.Exception.InnerExceptions);
    else if (first.IsCanceled) tcs.TrySetCanceled();
    else
    {
      try
      {
        next().ContinueWith(__ =>
        {
          if (t.IsFaulted) tcs.TrySetException(t.Exception.InnerExceptions);
          else if (t.IsCanceled) tcs.TrySetCanceled();
          else tcs.TrySetResult(null);
        }, TaskContinuationOptions.ExecuteSynchronously);
      }
      catch (Exception exc) { tcs.TrySetException(exc); }
    }
  }, TaskContinuationOptions.ExecuteSynchronously);
  return tcs.Task; 
}

你可以这样使用它：

var task = FooAsync().Then(() => BarAsync()).Then(() => FubarAsync());
task.ContinueWith(t =>
{
  if (t.IsFaulted || t.IsCanceled)
  {
    var e = t.Exception.InnerException;
    // exception handling
  }
  else
  {
    Console.WriteLine("All done");
  }
}, TaskContinuationOptions.ExcecuteSynchronously);

使用Rx，它看起来像这样（假设您没有async方法已经公开为IObservable<Unit>）：

FooAsync().ToObservable()
    .SelectMany(_ => BarAsync().ToObservable())
    .SelectMany(_ => FubarAsync().ToObservable())
    .Subscribe(_ => { Console.WriteLine("All done"); },
        e => { Console.WriteLine(e); });

我想。无论如何，我不是Rx大师。：）

Answer 2

为了完整起见，我将如何实施Chris Sinclair建议的辅助方法：

public void RunSequential(Action onComplete, Action<Exception> errorHandler,
                          params Func<Task>[] actions)
{
    RunSequential(onComplete, errorHandler,
                  actions.AsEnumerable().GetEnumerator());
}

public void RunSequential(Action onComplete, Action<Exception> errorHandler,
                          IEnumerator<Func<Task>> actions)
{
    if(!actions.MoveNext())
    {
        onComplete();
        return;
    }

    var task = actions.Current();
    task.ContinueWith(t => errorHandler(t.Exception),
                      TaskContinuationOptions.OnlyOnFaulted);
    task.ContinueWith(t => RunSequential(onComplete, errorHandler, actions),
                      TaskContinuationOptions.OnlyOnRanToCompletion);
}

这确保了只有在前一个任务成功完成后才会请求每个后续任务它假定Func<Task>返回已经运行的任务。

Answer 3

这里的内容基本上是ForEachAsync。您希望按顺序运行每个异步项，但需要一些错误处理支持。这是一个这样的实现：

public static Task ForEachAsync(IEnumerable<Func<Task>> tasks)
{
    var tcs = new TaskCompletionSource<bool>();

    Task currentTask = Task.FromResult(false);

    foreach (Func<Task> function in tasks)
    {
        currentTask.ContinueWith(t => tcs.TrySetException(t.Exception.InnerExceptions)
            , TaskContinuationOptions.OnlyOnFaulted);
        currentTask.ContinueWith(t => tcs.TrySetCanceled()
                , TaskContinuationOptions.OnlyOnCanceled);
        Task<Task> continuation = currentTask.ContinueWith(t => function()
            , TaskContinuationOptions.OnlyOnRanToCompletion);
        currentTask = continuation.Unwrap();
    }

    currentTask.ContinueWith(t => tcs.TrySetException(t.Exception.InnerExceptions)
            , TaskContinuationOptions.OnlyOnFaulted);
    currentTask.ContinueWith(t => tcs.TrySetCanceled()
            , TaskContinuationOptions.OnlyOnCanceled);
    currentTask.ContinueWith(t => tcs.TrySetResult(true)
            , TaskContinuationOptions.OnlyOnRanToCompletion);

    return tcs.Task;
}

我也支持取消任务，只是为了更一般，因为它花了很少的事情。

它将每个任务添加为上一个任务的延续，并且它确保任何异常都会导致最终任务的异常被设置。

以下是一个示例用法：

public static Task FooAsync()
{
    Console.WriteLine("Started Foo");
    return Task.Delay(1000)
        .ContinueWith(t => Console.WriteLine("Finished Foo"));
}

public static Task BarAsync()
{
    return Task.Factory.StartNew(() => { throw new Exception(); });
}

private static void Main(string[] args)
{
    List<Func<Task>> list = new List<Func<Task>>();

    list.Add(() => FooAsync());
    list.Add(() => FooAsync());
    list.Add(() => FooAsync());
    list.Add(() => FooAsync());
    list.Add(() => BarAsync());

    Task task = ForEachAsync(list);

    task.ContinueWith(t => Console.WriteLine(t.Exception.ToString())
        , TaskContinuationOptions.OnlyOnFaulted);
    task.ContinueWith(t => Console.WriteLine("Done!")
        , TaskContinuationOptions.OnlyOnRanToCompletion);
}

Answer 4

您应该能够创建一个方法来组合两个任务，并且只有在第一个成功时才开始第二个任务。

public static Task Then(this Task parent, Task next)
{
    TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
    parent.ContinueWith(pt =>
    {
        if (pt.IsFaulted)
        {
            tcs.SetException(pt.Exception.InnerException);
        }
        else
        {
            next.ContinueWith(nt =>
            {
                if (nt.IsFaulted)
                {
                    tcs.SetException(nt.Exception.InnerException);
                }
                else { tcs.SetResult(null); }
            });
            next.Start();
        }
    });
    return tcs.Task;
}

然后，您可以将任务链接在一起：

Task outer = FooAsync()
    .Then(BarAsync())
    .Then(FubarAsync());

outer.ContinueWith(t => {
    if(t.IsFaulted) {
        //handle exception
    }
});

如果您的任务立即开始，您可以将它们包装在Func：

中

public static Task Then(this Task parent, Func<Task> nextFunc)
{
    TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();
    parent.ContinueWith(pt =>
    {
        if (pt.IsFaulted)
        {
            tcs.SetException(pt.Exception.InnerException);
        }
        else
        {
            Task next = nextFunc();
            next.ContinueWith(nt =>
            {
                if (nt.IsFaulted)
                {
                    tcs.SetException(nt.Exception.InnerException);
                }
                else { tcs.SetResult(null); }
            });
        }
    });
    return tcs.Task;
}

Answer 5

现在，我还没有真正使用过TPL，所以这只是在黑暗中刺伤。考虑到@Servy提到的内容，也许这不会完全异步运行。但我想我会发布它，如果它方式没有标记，你可以向我遗忘或者我可以删除它（或者我们可以修复需要修复的东西）

public void RunAsync(Action onComplete, Action<Exception> errorHandler, params Action[] actions)
{
    if (actions.Length == 0)
    {
        //what to do when no actions/tasks provided?
        onComplete();
        return;
    }

    List<Task> tasks = new List<Task>(actions.Length);
    foreach(var action in actions)
    {
        Task task = new Task(action);
        task.ContinueWith(t => errorHandler(t.Exception), TaskContinuationOptions.OnlyOnFaulted);
        tasks.Add(task);
    }

    //last task calls onComplete
    tasks[actions.Length - 1].ContinueWith(t => onComplete(), TaskContinuationOptions.OnlyOnRanToCompletion);

    //wire all tasks to execute the next one, except of course, the last task
    for (int i = 0; i <= actions.Length - 2; i++)
    {
        var nextTask = tasks[i + 1];
        tasks[i].ContinueWith(t => nextTask.Start(), TaskContinuationOptions.OnlyOnRanToCompletion);
    }

    tasks[0].Start();
}

它的用法如下：

RunAsync(() => Console.WriteLine("All done"),
            ex => Console.WriteLine(ex),
            Foo,
            Bar,
            Fubar);

思考？ Downvotes？：）

（我当然更喜欢async / await）

编辑：根据你的评论Func<Task>，这是一个正确的实施吗？

public void RunAsync(Action onComplete, Action<Exception> errorHandler, params Func<Task>[] actions)
{
    if (actions.Length == 0)
    {
        //what to do when no actions/tasks provided?
        onComplete();
        return;
    }

    List<Task> tasks = new List<Task>(actions.Length);
    foreach (var action in actions)
    {
        Func<Task> nextActionFunc = action;
        Task task = new Task(() =>
        {
            var nextTask = nextActionFunc();
            nextTask.ContinueWith(t => errorHandler(t.Exception), TaskContinuationOptions.OnlyOnFaulted);
            nextTask.Start();
        });
        tasks.Add(task);
    }

    //last task calls onComplete
    tasks[actions.Length - 1].ContinueWith(t => onComplete(), TaskContinuationOptions.OnlyOnRanToCompletion);

    //wire all tasks to execute the next one, except of course, the last task
    for (int i = 0; i <= actions.Length - 2; i++)
    {
        var nextTask = tasks[i + 1];
        tasks[i].ContinueWith(t => nextTask.Start(), TaskContinuationOptions.OnlyOnRanToCompletion);
    }

    tasks[0].Start();
}

异步任务的顺序处理

5 个答案: