c# - 等待异步操作

Question

我有一个静态泛型方法，旨在并行地对集合执行操作(不用担心节流；我已经解决了这个问题；这里的代码是精简版):

public static IEnumerable<Task> RunAllAsync<T>(this IEnumerable<T> enumerable, Action<T> action)
{
    var tasks = new List<Task>();
    foreach (var t in enumerable)
    {
        tasks.Add(Task.Run(() => action(t)));
    }
    return tasks;
}

这是一个用法示例:

public async Task UsageSync()
{
    var numbers = Enumerable.Range(1, 5);
    var random = new Random();
    var tasks = numbers.RunAllAsync(i =>
                                    {
                                        Console.WriteLine($"A {i}");
                                        Thread.Sleep(random.Next(1000));
                                        Console.WriteLine($"B {i}");
                                    });
    Console.WriteLine("Awaiting end");
    await Task.WhenAll(tasks.ToArray());
    Console.WriteLine("awaited");
    Thread.Sleep(2000);
    Console.WriteLine("Finished");
}

输出:

Awaiting end
A 4
A 5
A 1
A 2
A 3
B 3
B 2
B 1
B 5
B 4
awaited
Finished

一切如预期。但现在我们使该操作本身成为一个异步委托(delegate):

public async Task UsageAsync()
{
    var numbers = Enumerable.Range(1, 5);
    var random = new Random();
    var tasks = numbers.RunAllAsync(async i =>
                                    {
                                        Console.WriteLine($"A {i}");
                                        await Task.Run(() =>
                                                       {
                                                           Console.WriteLine($"B {i}");
                                                           Thread.Sleep(random.Next(1000));
                                                           Console.WriteLine($"C {i}");
                                                       });
                                    });
    Console.WriteLine("Awaiting end");
    await Task.WhenAll(tasks.ToArray());
    Console.WriteLine("awaited");
    Thread.Sleep(2000);
    Console.WriteLine("Finished");
}

输出:

Awaiting end
A 4
B 4
A 1
A 5
A 3
A 2
B 2
awaited
B 5
B 1
B 3
C 3
C 2
C 1
C 5
C 4
Finished

看到了吗？该方法在线程完成之前达到“等待”状态!

我应该怎么做才能确保等待异步操作？

Answer 1

由于您正在处理 Action<T> , async i => { ... }被翻译成 async void方法。 async void设计使它不可能等待它完成。将您的参数更改为 Func<T, Task> func并更改调用 action(t)至 await func(t) .