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async和await,在main中应为非阻塞

asynchronous c#

arynaq · 技术社区 · 7 年前

从我对 async 关键字的用法 await in combination用于在实际需要异步操作的结果时创建一个延续点,允许在此期间完成其他工作。

那为什么下面的阻塞呢?我希望 Nothing to do while the awaits complete, expecting this line to come first. 作为控制台的第一行输出。。

tasks.cs

public static async Task Execute()
{
    var sw = new Stopwatch();
    sw.Start();
    await Foo();
    sw.Stop();
    Console.WriteLine($"Execute completed in {sw.ElapsedMilliseconds}ms.");
}

private static async Task Foo()
{
    var tasks = Enumerable.Range(0, 5).Select(x =>
    {
        return Task.Factory.StartNew((b) =>
        {
            Thread.Sleep(100);
            int value = (int) b;
            Console.WriteLine($"Task ran on thread: {Thread.CurrentThread.ManagedThreadId}");
            return value * value;
        }, x);
    }).ToArray();

    await Task.WhenAll(tasks);
}

它被称为main

static async Task Main(string[] args)
{
    await Tasks.Execute();
    var result = await LongRunningOperation();
    Console.WriteLine("Nothing to do while the awaits complete, expecting this line to come first.");
    Console.WriteLine($"Long running operation result: {result}");
}

private static async Task<int> LongRunningOperation()
{
    var sw = new Stopwatch();
    sw.Start();
    var res = await Task.Factory.StartNew(() =>
    {
        Thread.Sleep(10000);
        Console.WriteLine($"Long running operation completed on thread {Thread.CurrentThread.ManagedThreadId}");
        return 10000;
    });
    sw.Stop();

    return res;
}

其输出如下:

Task ran on thread: 7
Task ran on thread: 4
Task ran on thread: 3
Task ran on thread: 5
Task ran on thread: 6
Execute completed in 113ms.
Long running operation completed on thread 9
Nothing to do while the awaits complete, expecting this line to come first.
Long running operation result: 10000

这意味着我在这个上下文中阻塞,所有的东西都是按顺序链接在一起的。。。我不明白什么?

1 回复 | 直到 7 年前

A Friend 7 年前

从 Microsoft Docs :

通过写作 var result = await LongRunningOperation(); LongRunningOperation 已完成。

如果你重写你的 Main 看起来像这样:

static async Task Main(string[] args)
{
    var longTask = LongRunningOperation();
    Console.WriteLine("Nothing to do while the awaits complete, expecting this line to come first.");
    var result = await longTask;
    Console.WriteLine($"Long running operation result: {result}");
}

然后等待任务完成,然后再尝试输出结果。