Run two async tasks in parallel and collect results in .NET 4.5

While your Sleep method is async, Thread.Sleep is not. The whole idea of async is to reuse a single thread, not to start multiple threads. Because you've blocked using a synchronous call to Thread.Sleep, it's not going to work.

I'm assuming that Thread.Sleep is a simplification of what you actually want to do. Can your actual implementation be coded as async methods?

If you do need to run multiple synchronous blocking calls, look elsewhere I think!

You should use Task.Delay instead of Sleep for async programming and then use Task.WhenAll to combine the task results. The tasks would run in parallel.

public class Program
    {
        static void Main(string[] args)
        {
            Go();
        }
        public static void Go()
        {
            GoAsync();
            Console.ReadLine();
        }
        public static async void GoAsync()
        {

            Console.WriteLine("Starting");

            var task1 = Sleep(5000);
            var task2 = Sleep(3000);

            int[] result = await Task.WhenAll(task1, task2);

            Console.WriteLine("Slept for a total of " + result.Sum() + " ms");

        }

        private async static Task<int> Sleep(int ms)
        {
            Console.WriteLine("Sleeping for {0} at {1}", ms, Environment.TickCount);
            await Task.Delay(ms);
            Console.WriteLine("Sleeping for {0} finished at {1}", ms, Environment.TickCount);
            return ms;
        }
    }

async Task<int> LongTask1() { 
  ...
  return 0; 
}

async Task<int> LongTask2() { 
  ...
  return 1; 
}

...
{
   Task<int> t1 = LongTask1();
   Task<int> t2 = LongTask2();
   await Task.WhenAll(t1,t2);
   //now we have t1.Result and t2.Result
}