.Net Core异步编程

一. 用async关键字修饰的方法

1. 异步方法的返回值一般是Task,T是真正的返回值类型,Task.惯例:异步方法名字最好以Async结尾,这样方便我们只要是async
2. 即使方法没有返回值,也最好吧返回值声明为非泛型的Task.
3. 调用泛型方法时,一般在方法前加上await关键字,这样拿到的返回值就是泛型指定的T类型;
4. 异步方法的"传染性": 一个方法中如果await调用,则这个方法也必须修饰为async

static async Task Main(string[] args)
{
	string fileName = "d:/1.txt";
    File.Delete(fileName);
    File.WriteAllTextAsync(fileName,"hello async");
    string s = await File.ReadAllTextAsync(fileName);
    Console.WriteLine(s);
}

static async Task Main(string[] args)
{
    /*string fileName = @"F:\a\1.txt";
            File.WriteAllText(fileName, "hello");
            string s = File.ReadAllText(fileName);
            Console.WriteLine(s);*/
    string fileName = @"F:\a\1.txt";
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < 10000; i++)
    {
        sb.AppendLine("hello");
    }
    //这里如果不加await,会显示异常 因为还没写入完成就开始读取,读取是独占的
    await File.WriteAllTextAsync(fileName, sb.ToString());

    string s = await File.ReadAllTextAsync(fileName);

    //Task<string> t = File.ReadAllTextAsync(fileName);
    //string s = await t;
    Console.WriteLine(s);
}

如果同样的功能,既有同步方法,又有异步方法,那么首先使用异步方法. .Net5中,很多框架的方法也都支持异步:Main,WinForm时间处理函数.

对于不支持异步方法该怎么办?

Wait()(无返回值); Result(有返回值) 风险:死锁,尽量不用

static async Task Main(string[] args)
{
    int l = await DownloadHtmlAsync("https://www.youzack.com", @"F:\a\1.txt");
    Console.WriteLine("ok" + l);
}

/*static async Task DownloadHtmlAsync(string url, string fileame)
        {
            using (HttpClient httpClient = new HttpClient())
            {
                string html = await httpClient.GetStringAsync(url);
                await File.WriteAllTextAsync(fileame, html);
            }
        }*/

static async Task<int> DownloadHtmlAsync(string url, string fileame)
{
    using (HttpClient httpClient = new HttpClient())
    {
        string html = await httpClient.GetStringAsync(url);
        await File.WriteAllTextAsync(fileame, html);
        return html.Length;
    }
}

二. await关键字修饰的方法

await调用的等待期间,.NET会把当前的线程返回给线程池,等异步方法调用执行完毕后,框架会从线程池在取出来一个线程执行后续的代码.

Thread.CurrentThread.ManageThreadId 获得当前线程Id

验证:在耗时异步(写入大字符串)操作前后分别打印线程Id

static async Task Main(string[] args)
{
    Console.WriteLine(Thread.CurrentThread.ManagedThreadId);
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < 1000; i++)
    {
        sb.Append("XXXXXXXXXXXXXXXXX");
    }

    await File.WriteAllTextAsync(@"F:\a\1.txt", sb.ToString());
    Console.WriteLine(Thread.CurrentThread.ManagedThreadId);
}

优化:到要等待的时候,如果发现已经执行结束了,那就没必要再切换线程了,剩下的代码就继续在之前的线程上就行执行了

三.异步方法不等于多线程

static async Task Main(string[] args)
{
    Console.WriteLine("之前" + Thread.CurrentThread.ManagedThreadId);
    double r = await CalcAsync(5000);
    Console.WriteLine($"r = {r}");
    Console.WriteLine("之后" + Thread.CurrentThread.ManagedThreadId);
}

public static async Task<double> CalcAsync(int n)
{
    Console.WriteLine("CalcAsync," + Thread.CurrentThread.ManagedThreadId);
    double result = 0;
    Random rand = new Random();
    for (int i = 0; i < n*n; i++)
    {
        result += rand.NextDouble();
    }
    return result;
}

结论: 异步方法的代码并不会自动在新线程中执行,除非把代码放到新线程中执行.

把要执行的代码以委托的形式传递给Task().Run().这样就会从线程池中出去一个线程执行我们的委托

await Task.Run(()=>{
    //耗时操作代码,可以返回return返回值
})