C#多线程之await/async

await/async：是由.net4.5引入的语法糖。（由编译器提供的便捷功能就是语法糖）
await/async语法：
1.）async 是用来修饰方法，如果单独出现，方法会警告,没有什么作用；
2.）await在方法体内部，只能放在async修饰的方法内，必须放在task前面；
3.）async/await方法里面如果没有返回值，默认返回一个Task，或者void(推荐用Task，而不是void，因为这样才能await/wait)；
4.）带async+await后，返回值要多一层Task<>；

1.使用Task，Async，Sync读取文件各个需要耗时时间：

#region ReadFile
public static void Show()
{
    {
        Console.WriteLine("******************ReadFile***************");
        string path = "D:\\ceshi\\ceshi.zip";
        int loopNum = 10;//30
        {
            Console.WriteLine("*****************Async****************");
            List<Task> taskList = new List<Task>();
            Stopwatch stopwatch = new Stopwatch();
            stopwatch.Start();
            for (int i = 0; i < loopNum; i++)
            {
                taskList.Add(ReadAsync(path, i));
            }
            Task.WaitAll(taskList.ToArray());
            stopwatch.Stop();
            Console.WriteLine($"Async耗时{stopwatch.ElapsedMilliseconds}ms,ThreadId={Thread.CurrentThread.ManagedThreadId}")
        }
        Thread.Sleep(3000);
        {
            Console.WriteLine("*****************Task****************");
            List<Task> taskList = new List<Task>();
            Stopwatch stopwatch = new Stopwatch();
            stopwatch.Start();
            for (int i = 0; i < loopNum; i++)
            {
                taskList.Add(ReadTask(path, i));
            }
            Task.WaitAll(taskList.ToArray());
            stopwatch.Stop();
            Console.WriteLine($"Task耗时{stopwatch.ElapsedMilliseconds}ms,ThreadId={Thread.CurrentThread.ManagedThreadId}");
        }
        Thread.Sleep(3000);
        {
            Console.WriteLine("*****************Sync****************");
            Stopwatch stopwatch = new Stopwatch();
            stopwatch.Start();
            for (int i = 0; i < loopNum; i++)
            {
                ReadSync(path, i);
            }
            stopwatch.Stop();
            Console.WriteLine($"Sync耗时{stopwatch.ElapsedMilliseconds}ms,ThreadId={Thread.CurrentThread.ManagedThreadId}");
        }
    }
#endregion
#region Read
private static async Task<byte[]> ReadAsync(string path, int num)
{
    Console.WriteLine($"This is ReadAsync{num} Start,ThreadId={Thread.CurrentThread.ManagedThreadId}");
    var result = await File.ReadAllBytesAsync(path);
    Console.WriteLine($"This is ReadAsync{num}   End,ThreadId={Thread.CurrentThread.ManagedThreadId}");
    return result;
}
private static Task<byte[]> ReadTask(string path, int num)
{
    Console.WriteLine($"This is ReadTask{num} Start,ThreadId={Thread.CurrentThread.ManagedThreadId}");
    var result = Task.Run(() =>
    {
        Console.WriteLine($"This is ReadTask Ing,ThreadId={Thread.CurrentThread.ManagedThreadId}");
        return File.ReadAllBytes(path);
    });
    Console.WriteLine($"This is ReadTask{num}   End,ThreadId={Thread.CurrentThread.ManagedThreadId}");
    return result;
}
private static byte[] ReadSync(string path, int num)
{
    Console.WriteLine($"This is ReadSync{num} Start,ThreadId={Thread.CurrentThread.ManagedThreadId}");
    var result = File.ReadAllBytes(path);
    Console.WriteLine($"This is ReadSync{num}   End,ThreadId={Thread.CurrentThread.ManagedThreadId}");
    return result;
}
#endregion

从上面的代码可以看到Async在读取方法是调用的是“File.ReadAllBytesAsync”方法,而该方法大致采用DMA技术，而该技术实现原理是，硬盘接受命令后，CPU继续执行自己的线程，当硬盘中“读/写”自己操作完毕后会发中断信号，CPU会继续处理，而这中间就是基于DMA异步操作，从而节省CPU资源。
2.适用场景
1.）跟第三方交互（非托管资源，Async版本）
2.）数据库OpenAsync
3.）Redis
4.）Web请求
5.）文件读取