在.net中管理异步任务

Question

I am creating program to download some files. I am testing the async methods and I have problem. I want to set limit one the amount of downloaded files. I have static method:

public static async Task StreamToFile(Stream input, string fileName)
    {
        if (input != null)
        {
            string dir = Path.GetDirectoryName(fileName);
            if (!Directory.Exists(dir))
                Directory.CreateDirectory(dir);
            using (Stream str = File.Create(fileName))
            {
                await input.CopyToAsync(str);
                input.Dispose();
            }
        }
    }

Stream is created outside. And now, I want to call this method for example 10 times, then application should wait for completing any of running task and start new one. I don't want to wait for all ten tasks, I want to start new immediately after one of the tasks close. How can I manage tasks? In Threads I was able to create old school List with threads, set while loop

while(ListOfThreads.Count >=Limit)
{
    for(int i=0;i<ListOfThreads.Count;i++)
    {
        if (ListOfThreads[i].IsCompleted)
        {
            ListOfThreads.RemoveAt(i);
            i--;
        }
    }
}

but when I try to do the same with Tasks program stucks and never reaches input.Dispose() in StreamToFile. Tasks have WaitingForActivation status.

When I remove loop everything works fine but of course program are downloading all links that I paste

Answer 1

SemaphoreSlim can be used to throttle asynchronous operations:

private static SemaphoreSlim mutex = new SemaphoreSlim(10);
public static async Task StreamToFile(Stream input, string fileName)
{
  await mutex.WaitAsync();
  try
  {
    if (input != null)
    {
      string dir = Path.GetDirectoryName(fileName);
      if (!Directory.Exists(dir))
        Directory.CreateDirectory(dir);
      using (Stream str = File.Create(fileName))
      {
        await input.CopyToAsync(str);
        input.Dispose();
      }
    }
  }
  finally
  {
    mutex.Release();
  }
}

Answer 2

I looked it for 3 hours but after posting question I found solution: https://msdn.microsoft.com/pl-pl/library/JJ155756.aspx

I have to create list of tasks

List<Task> tasksList= new List<Task>();

and check if it reaches the limit

if(tasksList.Count>=limit)
{
    var firstFinished= await Task.WhenAny(tasksList);
    tasksList.Remove(firstFinished);
}

then Add of course task

tasksList.Add(AsyncMethod(parameters));