为什么分配任务然后等待它允许并行运行

Question

我一直在玩异步，我遇到了一些我以前没有注意到的行为，如果这是重复的请告诉我，但是我的 google-fu 失败了，主要是因为我想不出合适的搜索词：

给定一个执行一些参数化工作的简单异步方法：

async Task<String> Foo(int i)
{
    await Task.Delay(i);
    return i.ToString();
}

以及在不同上下文中调用它并捆绑结果的调用方法：

async Task<Object> Bar()
{
    var one =    Foo(3000);
    var two =    Foo(5000);
    var three =  Foo(3000);

    var x =
        new
        {
            One =   await one,
            Two =   await two,
            Three = await three,
        };      
        
    return x;
}

这在 5 秒内完成（在 Linqpad6、.NET Core 3.1 中）。所以我假设每个任务同时运行。

但是，如果我将其更改为在开始时等待，它会在 11 秒内完成。所以我假设每个任务按顺序运行。

async Task<Object> Bar()
{
    var one =   await Foo(3000);
    var two =   await Foo(5000);
    var three = await Foo(3000);

    var x =
        new
        {
            One =   one,
            Two =   two,
            Three = three,
        };
        
    return x;
}

我的问题是，分配任务然后等待它与仅等待允许他们并行完成的任务有什么区别？

Answer 1

查看行内评论

async Task<Object> Bar()
{
    var one =    Foo(3000); // <-- It can Start
    var two =    Foo(5000); // <-- It can Start
    var three =  Foo(3000); // <-- It can Start

    var x =
        new
        {
            One =   await one,  // <-- you are waiting for it to finish
            Two =   await two,  // <-- you are waiting for it to finish
            Three = await three,// <-- you are waiting for it to finish
        };      
        
    return x;
}

async Task<Object> Bar()
{
    var one =   await Foo(3000); // <-- you are waiting for it to finish
    var two =   await Foo(5000); // <-- you are waiting for it to finish
    var three = await Foo(3000); // <-- you are waiting for it to finish

    var x =
        new
        {
            One =   one,
            Two =   two,
            Three = three,
        };
        
    return x;
}

为什么分配任务然后等待它允许并行运行

Why does assigning a Task then awaiting it allow to run in parallel

c#

async-await

task-parallel-library

为什么分配任务然后等待它允许并行 运行

Why does assigning a Task then awaiting it allow to run in parallel

c#

async-await

task-parallel-library

为什么分配任务然后等待它允许并行运行