异步方法同步运行c#

我试图编写一个异步方法,以便在一组较大的日期之间为一组日期执行一些逻辑,但它不是异步运行的。我试着在这段时间里 startDate 和 endDate ,但如果要计算的时间少于5天,则以5个或更少的块为单位。

所以从 6-1-2018 到 6-29-2018 总共有5个块要进行逻辑操作。我试图同时为每个块触发一个异步方法。

执行每个任务的总时间应该在5秒左右,因为任务只是休眠5秒。它目前正在运行30秒,这意味着它 不 异步,但同步。我如何修复它以便可以异步运行这些操作?

public async static void Foo()
{
    var watch = System.Diagnostics.Stopwatch.StartNew();

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

    DateTime endDate = new DateTime(2018, 6, 29);
    int dayIntervals = 4; // Query 5 days at a time

    for (DateTime startDate = new DateTime(2018, 6, 1); startDate.Date <= endDate.Date; startDate = startDate.AddDays(dayIntervals + 1))
    {
        dayIntervals = (startDate.AddDays(dayIntervals) > endDate ? dayIntervals = endDate.Day - startDate.Day : dayIntervals);
        tasks.Add(Action(startDate, startDate.AddDays(dayIntervals))); // Add the tasks
    }

    await Task.WhenAll(tasks); // Fire them asynchronously?
    watch.Stop();
    var elapsed = TimeSpan.FromMilliseconds(watch.ElapsedMilliseconds).TotalSeconds;

    Debug.WriteLine("Finished in: " + elapsed + " seconds"); // 30 sec
}

public static Task<DataTable> Action(DateTime one, DateTime two)
{
    // Junk related to the table to be returned
    // ...

    Random r = new Random();
    Thread.Sleep(5000);

    return Task.FromResult(table);
}