C# Async,Await简单理解（应用层次）

这里我就不给大家去研究这两上下文关键字的il实现了，感兴趣的朋友可以自己去用ildasm分析一下。
我写这篇文章只是想强调微软官方文档上写的一点：

async和await并没有开辟新的线程，我们来分别执行一下io密集型和cpu密集型代码

：

static void Main(string[] args){
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
GetUrlContentLengthAsync();
stopwatch.Stop();
Console.WriteLine("async,await时间： " + stopwatch.ElapsedMilliseconds);
 
Stopwatch stopwatch1 = new Stopwatch();
stopwatch1.Start();
GetUrlContentLength();
stopwatch1.Stop();
Console.WriteLine("非async,await时间： " + stopwatch1.ElapsedMilliseconds);
}
 
public async static Task<int> GetUrlContentLength()
{
var client = new HttpClient();
 
string getStringTask =
client.GetStringAsync("https://learn.microsoft.com/dotnet").ToString();
 
DoIndependentWork();
 
string contents = getStringTask;
 
return contents.Length;
}
 
public async static Task<int> GetUrlContentLengthAsync()
{
var client = new HttpClient();
 
Task<string> getStringTask =
client.GetStringAsync("https://learn.microsoft.com/dotnet");
 
DoIndependentWork();
 
string contents = await getStringTask;
 
return contents.Length;
}
 
static void DoIndependentWork()
{
Console.WriteLine("Working...");
}

运行结果：

这是io密集型，就是微软官方文档的示例代码
为什么用了这个反而更慢？我理解的是因为存在上下文状态的切换。
可能我这样说不是很直观，我推荐阅读微软原文档：
https://learn.microsoft.com/zh-cn/dotnet/csharp/asynchronous-programming/task-asynchronous-programming-model?source=recommendations

我们再来看一个cpu密集型操作：

static void Main(string[] args){
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
GetnumAsync();
stopwatch.Stop();
Console.WriteLine("async,await时间： " + stopwatch.ElapsedMilliseconds);
 
Stopwatch stopwatch1 = new Stopwatch();
stopwatch1.Start();
Getnum();
stopwatch1.Stop();
Console.WriteLine("主线程时间： " + stopwatch1.ElapsedMilliseconds);
}
public async static Task<int> GetnumAsync()
{
int num = 0;
await Task.Run(() =>
{
for (int i = 0; i < 100000000; i++)
{
num += i;
}
});
 
return num;
}
 
public static int Getnum()
{
Stopwatch stopwatch1 = new Stopwatch();
stopwatch1.Start();
int num = 0;
for (int i = 0; i < 100000000; i++)
{
num += i;
}
stopwatch1.Stop();
Console.WriteLine("子线程时间： " + stopwatch1.ElapsedMilliseconds);
 
 
return num;
}