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

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!