C# async/await strange behavior in console app

Question

I built some async/await demo console app and get strange result. Code:

class Program
{
    public static void BeginLongIO(Action act)
    {
        Console.         


        

Answer 1

the line that actually runs something on a background thread is

   Task.Run( () => {  } ); 

In your example you are not awaiting that Task but that of a TaskCompletionSource

   public static Task LongIOAsync()
   {
        var tcs = new TaskCompletionSource();

        Task.Run ( () => BeginLongIO(() =>
        {
            try { tcs.TrySetResult(EndLongIO()); }
            catch (Exception exc) { tcs.TrySetException(exc); }
        }));

        return tcs.Task;    
   }

When awaiting LongIOAsync you are awaiting the the task from tcs witch is set from a background thread in a delegate given to Task.Run()

apply this change :

    public static Task LongIOAsync()
    {
        Console.WriteLine("In LongIOAsync start... {0} {1}", (DateTime.Now.Ticks - ticks) / TimeSpan.TicksPerMillisecond, Thread.CurrentThread.ManagedThreadId);
        var tcs = new TaskCompletionSource();

        Task.Run ( () => BeginLongIO(() =>
        {
            try { tcs.TrySetResult(EndLongIO()); }
            catch (Exception exc) { tcs.TrySetException(exc); }
        }));

        Console.WriteLine("In LongIOAsync end... \t{0} {1}", (DateTime.Now.Ticks - ticks) / TimeSpan.TicksPerMillisecond, Thread.CurrentThread.ManagedThreadId);
        return tcs.Task;
    }

Alternately in this case you could of just awaited the returned from Task.Run() , TaskCompletionSource is for situations where you want to pass around the ability to set your Task as complete or other wise.