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

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

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;
    }
}