Task 异步小技巧
source link: https://www.cnblogs.com/ysmc/p/15409898.html
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Task 异步小技巧
原文地址：Task 异步小技巧 - 一事冇诚 - 博客园 (cnblogs.com)
async Task 语法糖出来后，异步编程变得非常简单，适合需要耗费较长时间的任务。
有些小伙伴使用后可能会非常疑惑，使用异步和同步，在耗时上几乎没有差别。
下面我们看一个例子，场景是需要调用多个第三方的WebApi，分别是获取名称、年龄、性别，由于网络环境等原因，api响应时间可能会接近1秒
1 public async Task Test()
2 {
3 var sw = new Stopwatch();
4 sw.Start();
5
6 var userName = await GetUserNameAsync();
7 var userAge = await GetUserAgeAsync();
8 var userSex = await GetUserSexAsync();
9
10 sw.Stop();
11 var ts = sw.Elapsed;
12 Console.WriteLine($"总共耗时：{ts.TotalMilliseconds}ms");
13 }
14
15 private async Task<string> GetUserNameAsync()
16 {
17 await Task.Delay(500);
18 return "小明";
19 }
20
21 private async Task<string> GetUserAgeAsync()
22 {
23 await Task.Delay(800);
24 return "11";
25 }
26
27 private async Task<string> GetUserSexAsync()
28 {
29 await Task.Delay(900);
30 return "11";
31 }
运行后发现，这个时间2秒多，这用户体验肯定是无法忍受的
导致这样结果的原因是每次进行异步调用的时候，都在异步函数前加上了 await ，这会导致该线程阻塞，等待直到结果返回，每个异步函数都await，时间自然就叠加了，为了解决这个问题，使用一个小技巧，可以将代码改成下面这样
1 public async Task Test()
2 {
3 var sw = new Stopwatch();
4 sw.Start();
5
6 var userNameTask = GetUserNameAsync();
7 var userAgeTask = GetUserAgeAsync();
8 var userSexTask = GetUserSexAsync();
9
10 var userName = await userNameTask;
11 var userAge = await userAgeTask;
12 var userSex = await userSexTask;
13
14 sw.Stop();
15 var ts = sw.Elapsed;
16 Console.WriteLine($"总共耗时：{ts.TotalMilliseconds}ms");
17 }
18
19 private async Task<string> GetUserNameAsync()
20 {
21 await Task.Delay(500);
22 return "小明";
23 }
24
25 private async Task<string> GetUserAgeAsync()
26 {
27 await Task.Delay(800);
28 return "11";
29 }
30
31 private async Task<string> GetUserSexAsync()
32 {
33 await Task.Delay(900);
34 return "11";
35 }
这次运行的总耗时，就是3个异步中，耗时最长那个 GetUserSexAsync
为什么会这样呢，这个小技巧的关键是这里，当执行到异步函数的时候，不加 await，不进行等待，这样就不会造成阻塞，让这些任务乖乖在别的线程的执行，当需要用到他们的时候，再去等待返回值，所以时间上不会进行叠加，哪个最长，总耗时就是哪个
1 var userNameTask = GetUserNameAsync(); 2 var userAgeTask = GetUserAgeAsync(); 3 var userSexTask = GetUserSexAsync(); 4 5 var userName = await userNameTask; 6 var userAge = await userAgeTask; 7 var userSex = await userSexTask;
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