正确使用Async/Await的困惑

Question

这是我在Windows Forms Application中实现的async/await

async Task<int> DoAysnc1()
{
    await Task.Delay(3000);
    return 3000;
}

async Task<int> DoAsync2()
{
    await Task.Delay(5000);
    return 5000;
}

private async void button1_Click(object sender, EventArgs e)
{
    this.textBox1.Text = "";
    var doAsync1 = DoAysnc1();
    var doAsync2 = DoAysnc2();
    var async1 = await doAsync1;
    var async2 = await doAsync2;
    this.textBox1.Text = $"{async1} & {async2}";
}

5秒后,TextBox中的结果为" 3000 & 5000".

但是,当我button1_Click像这样修改:

private async void button1_Click(object sender, EventArgs e)
{
    this.textBox1.Text = "";
    var async1 = await DoAysnc1();
    var async2 = await DoAysnc2();
    this.textBox1.Text = $"{async1} & {async2}";
}

结果相同,但需要8秒.

为什么第二版的button1_Click行为同步？

Answer 1

以下是对差异的解释:

this.textBox1.Text = "";
var doAsync1 = DoAysnc1();         // <--- Run DoAsync1
var doAsync2 = DoAysnc2();         // <--- Run DoAsync2
var async1 = await doAsync1;       // <--- wait for DoAsync1 to finish
var async2 = await doAsync2;       //<--- wait for DoAsync2 to finish
this.textBox1.Text = $"{async1} & {async2}";

而:

this.textBox1.Text = "";
var async1 = await DoAysnc1();          // <-- Run DoAsync1 and wait for it to be done
var async2 = await DoAysnc2();          // <-- Run DoAsync2 and wait for it to be done
this.textBox1.Text = $"{async1} & {async2}";

因此在第一个版本中,两个任务都在同一时间运行.在第二个版本中,您从未运行第二个任务,直到第一个任务完成.

我认为阅读这篇文章对你的知识来说将是一个很大的好处.