异步并等待，期望在主程序中不阻塞

Question

根据我对async关键字的理解，结合使用await可以为实际需要异步操作的结果创建连续点，从而允许在此期间进行其他工作。

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. 是输出到控制台的第一行。

在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);
}

在主叫

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;
}

输出以下内容：

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

这意味着我在这种情况下处于阻塞状态，并且一切都按顺序链接在一起……我不明白什么？

Answer 1

从Microsoft Docs ：

将await运算符应用于异步方法中的任务，以在该方法的执行中插入一个暂停点，直到等待的任务完成

通过写var result = await LongRunningOperation(); 您将暂停任何进一步的操作，直到LongRunningOperation完成。

如果您将Main重写为如下所示：

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}");
}

然后您的程序将打印预期的行， 然后等待任务完成，然后再尝试输出结果。