等待不釋放異步方法中的調用線程

Question

我正在AsyncDemoViewModel中打印出Messages屬性字符串列表。

如果我在GetVideosSlowlyAsync（）中使用Thread.Sleep（1000），則輸出為

Time spent: 4001 milliseconds 

 - Started on thread 18, finished on thread 18  
 - Started on thread 18, finished on thread 18  
 - Started on thread 18, finished on thread 18 
 - Started on thread 18, finished on thread 18

如果我在GetVideosSlowlyAsync（）中使用await Task.Delay（1000），則輸出為

Time spent: 4053 milliseconds 


 - Started on thread 12, finished on thread 19 
 - Started on thread 19, finished on thread 16 
 - Started on thread 16, finished on thread 19 
 - Started on thread 19, finished on thread 9

為什么await調用不能釋放調用線程？ 我期望await版本的完成時間快4倍左右。

控制器代碼：

public class AsyncDemoController : Controller
{
    private DepartmentDb _db;

    public AsyncDemoController(DepartmentDb db)
    {
        _db = db; 
    }

    public async Task<ActionResult> Index()
    {

        var sw = Stopwatch.StartNew();
        var v1 = await GetVideosSlowlyAsync();
        var v2 = await GetVideosSlowlyAsync();
        var v3 = await GetVideosSlowlyAsync();
        var v4 = await GetVideosSlowlyAsync();

        var vm = new AsyncDemoViewModel() {Videos = v1.Item2, Messages = new List<string>()};
        sw.Stop();
        vm.Messages.Add(string.Format("Time spent: {0} milliseconds", sw.ElapsedMilliseconds));
        vm.Messages.Add(v1.Item1);
        vm.Messages.Add(v2.Item1);
        vm.Messages.Add(v3.Item1);
        vm.Messages.Add(v4.Item1);

        return View(vm);
    }

    private async Task<Tuple<string, IEnumerable<Video>>> GetVideosSlowlyAsync()
    {
        var t1 = Thread.CurrentThread.ManagedThreadId;
        await Task.Delay(1000); // Thread.Sleep(1000);
        var t2 = Thread.CurrentThread.ManagedThreadId;
        return Tuple.Create(string.Format("Started on thread {0}, finished on thread {1}", t1, t2), _db.Videos.AsEnumerable());
    }

}

Answer 1

等待方法等待GetVideosSlowlyAsync方法完成。 您需要將await移到需要操作結果的位置：

public async Task<ActionResult> Index()
{

    var sw = Stopwatch.StartNew();
    var v1 = GetVideosSlowlyAsync();
    var v2 = GetVideosSlowlyAsync();
    var v3 = GetVideosSlowlyAsync();
    var v4 = GetVideosSlowlyAsync();

    var vm = new AsyncDemoViewModel() {Videos = (await v1).Item2, Messages = new List<string>()};
    sw.Stop();
    vm.Messages.Add(string.Format("Time spent: {0} milliseconds", sw.ElapsedMilliseconds));
    vm.Messages.Add((await v1).Item1);
    vm.Messages.Add((await v2).Item1);
    vm.Messages.Add((await v3).Item1);
    vm.Messages.Add((await v4).Item1);

    return View(vm);
}

Answer 2

您正在調用await GetVideosSlowlyAsync() ，它將返回到調用方法，直到任務完成。 如果要查看結果，則期望刪除此等待狀態。 這樣，當Task.Delay()方法在線程池上休眠時， Task.Delay()一個接一個地啟動所有4個GetVideosSlowlyAsync() 。

試試這個

List<Task> taskList = new List<Task>();
taskList.Add(GetVideosSlowlyAsync());
taskList.Add(GetVideosSlowlyAsync());
taskList.Add(GetVideosSlowlyAsync());
taskList.Add(GetVideosSlowlyAsync());

Task.WaitAll(taskList.ToArray())
foreach (Task task in taskList)
    vm.Messages.Add(task.Result.Item1);