Run two async tasks in parallel and collect result

Question

I've been trying for a while to get something I thought would be simple working with .NET 4.5

I want to fire off two long running tasks at same time and collect the
results in in the best C# 4.5 (RTM) way

The following works but I don't like it because:

• I want Sleep to be an async method so it can await other methods
• It just looks clumsy with Task.Run()
• I don't think this is even using any new language features at all!

Working code:

public static void Go()
{
    Console.WriteLine("Starting");

    var task1 = Task.Run(() => Sleep(5000));    
    var task2 = Task.Run(() => Sleep(3000));

    int totalSlept = task1.Result + task2.Result;

    Console.WriteLine("Slept for a total of " + totalSlept + " ms");
}

private static int Sleep(int ms)
{
    Console.WriteLine("Sleeping for " + ms);
    Thread.Sleep(ms);
    Console.WriteLine("Sleeping for " + ms + " FINISHED");
    return ms;
}

Non working code:

Update: This actually works and is the correct way to do it, the only problem is the Thread.Sleep

This code doesn't work because the call to Sleep(5000) immediately starts the task running so Sleep(1000) doesn't run until it has completed. This is true even though Sleep is async and I'm not using await or calling .Result too soon.

I thought maybe there is a way to get a non-running Task<T> by calling an async method so I could then call Start() on the two tasks, but I can't figure out how to get a Task<T> from calling an async method.

public static void Go()
{
    Console.WriteLine("Starting");

    var task1 = Sleep(5000);    // blocks
    var task2 = Sleep(1000);

    int totalSlept = task1.Result + task2.Result;

    Console.WriteLine("Slept for " + totalSlept + " ms");
}

private static async Task<int> Sleep(int ms)
{
    Console.WriteLine("Sleeping for " + ms);
    Thread.Sleep(ms);
    return ms;
}

Answer 1

To answer this point:

I want Sleep to be an async method so it can await other methods

you can maybe rewrite the Sleep function like this:

private static async Task<int> Sleep(int ms)
{
    Console.WriteLine("Sleeping for " + ms);
    var task = Task.Run(() => Thread.Sleep(ms));
    await task;
    Console.WriteLine("Sleeping for " + ms + "END");
    return ms;
}

static void Main(string[] args)
{
    Console.WriteLine("Starting");

    var task1 = Sleep(2000);
    var task2 = Sleep(1000);

    int totalSlept = task1.Result +task2.Result;

    Console.WriteLine("Slept for " + totalSlept + " ms");
    Console.ReadKey();
}

running this code will output :

Starting
Sleeping for 2000
Sleeping for 1000
*(one second later)*
Sleeping for 1000END
*(one second later)*
Sleeping for 2000END
Slept for 3000 ms

Answer 2

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;
        }
    }

Answer 3

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!

Answer 4

It's weekend now!

public static void Go() {
    Console.WriteLine("Start fosterage...\n");
    var t1 = Sleep(5000, "Kevin");
    var t2 = Sleep(3000, "Jerry");
    var result = Task.WhenAll(t1, t2).Result;

    Console.WriteLine("\nMy precious spare time last for only {0}ms", result.Max());
    Console.WriteLine("Press any key and take same beer...");
    Console.ReadKey();
}

private static async Task<int> Sleep(int ms, string n) {
        Console.WriteLine("{0} going to sleep for {1}ms :)", n, ms);
        await Task.Delay(ms);
        Console.WriteLine("{0} waked up after {1}ms :(", n, ms);
        return ms;
}

Ordinary Task.Factory example:

   private static Task<int> Sleep(int ms, string n) {
        return Task.Factory.StartNew(() => {
            Console.WriteLine("{0} going to sleep for {1}ms :)", n, ms);
            Thread.Sleep(ms);
            Console.WriteLine("{0} waked up after {1}ms :(", n, ms);
            return ms;
        });
    }

Mysterious TaskCompletionSource example:

private static Task<int> Sleep(int ms, string n) {
    var tcs = new TaskCompletionSource<int>();
    Console.WriteLine("{0} going to sleep for {1}ms :)", n, ms);
    var t = Task.Factory.StartNew(() => {
        Thread.Sleep(ms);
        Console.WriteLine("{0} waked up after {1}ms :(", n, ms);
        tcs.SetResult(ms);
    });
    return tcs.Task;
}

Answer 5

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

Answer 6

This article helped explain a lot of things. It's in FAQ style.

Async/Await FAQ

This part explains why Thread.Sleep runs on the same original thread - leading to my initial confusion.

Does the “async” keyword cause the invocation of a method to queue to the ThreadPool? To create a new thread? To launch a rocket ship to Mars?

No. No. And no. See the previous questions. The “async” keyword indicates to the compiler that “await” may be used inside of the method, such that the method may suspend at an await point and have its execution resumed asynchronously when the awaited instance completes. This is why the compiler issues a warning if there are no “awaits” inside of a method marked as “async”.