Question

这是我在Windows窗体应用程序中实现的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＆＃34; 3000 & 5000＆＃34;。

但是当我像这样修改button1_Click时：

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button1_Click会同步？

Answer 1

以下是差异的解释：

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}";

虽然：

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}";

所以在第一个版本中，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Article对你的知识来说将是一个很大的优势。

Answer 2

通过一些评论可以使实际发生的事情更清楚：

// Starts DoAsync1 asynchronously
var doAsync1 = DoAysnc1();

// Starts DoAsync2 asynchronously
var doAsync2 = DoAysnc2();

// From now on, both task are executing

// Wait for DoAsync1 completion
var async1 = await doAsync1;

// Wait for DoAsync2 completion
var async2 = await doAsync2;

对于第二种情况：

// Starts DoAsync1 asynchronously, and wait for the task completion (3s)
var async1 = await DoAysnc1();

// Starts DoAsync2 asynchronously, and wait for the task completion (5s)
var async2 = await DoAysnc2();

Answer 3

DoAsync1和DoAsync2返回等待的Task个对象。如果你一个接一个地等待它们，那么它们将被一个接一个地执行（首先等待第一个完成，然后等待第二个）。为了并行运行它们，您可以先创建Task - 对象，然后使用Task.WhenAll等待其结果。

private async void button1_Click(object sender, EventArgs e)
{
    this.textBox1.Text = "";

    var task1 = DoAysnc1();
    var task2 = DoAysnc2();
    await Task.WhenAll(task1, task2)

    this.textBox1.Text = $"{task1.Result} & {task2.Result}";
}

正确使用Async / Await的困惑

3 个答案: