How to run multiple methods asynchron and in parallel with delay

Question

I'm trying to run two methods in parallel. The first method connects to an ftp server and downloads some data. Because I want to reduce the network traffic it should run every 30 s. In parallel I want another method to run independent from the first method every 10 s.

The problem is I don't get the methods running/delayed in parallel.

namespace Example
{
    class Program
    {
        static async Task Main(string[] args)
        {
            await Task.Run(async () =>
            {
                while (true)
                {
                    await Every10s();
                    await Every30s();
                }
            });
        }

        public static async Task<bool> Every10s()
        {
            await Task.Delay(10000);
            Console.Writeline("10s");
            return true;
        }

        public static async Task<bool> Every30s()
        {
            await Task.Delay(30000);            
            Console.Writeline("30s");
            return true;
        }
    }
}

I would expect the following output with the corresponding pauses in between: 10s 10s 10s 30s 10s 10s 10s 30s ...

But instead both methods wait for each other so I get the output 10s 30s 10s 30s 10s 30s with a 40s pause.

Any help and hints are appreciated.

Instead of awaiting the Tasks the method return, you should store them in a List and perform `await Task.WhenAll(list)` on it. You'd need to do that on each while-cycle. Note, for this use case I'd suggest using timers over async/await since this is literally the definiton of a timer. Another reason not to use async/await here is because async/await isn't truly parallel (I'd link to an awesome write up here but I forgot where it's from). — Joelius, Jun 24 '19 at 12:09
You are mixing up parallel execution and asynchronous execution. Async isn't parallel. — iSpain17, Jun 24 '19 at 12:10
How is this program going to end? There is no way of exiting the infinite loop! — Theodor Zoulias, Jun 24 '19 at 12:16
I forgot where the write up is from but there are so many sources on this like [this answer](https://stackoverflow.com/a/14099602/10883465), [this question](https://stackoverflow.com/questions/35126393/async-await-and-parallel) and many more if you search a bit. — Joelius, Jun 24 '19 at 12:16
@Joelius, thanks for pointing that out, I'll also give timers a try. — Pietr Podschinski, Jun 24 '19 at 12:42

score 4 · Accepted Answer · answered Jun 24 '19 at 12:13

4

Because I want to reduce the network traffic it should run every 30 s. In parallel I want another method to run independent from the first method every 10 s.

You have two independent loops of work, so you need two loops in your code:

async Task RunEvery10s()
{
  while (true)
    await Every10s();
}

async Task RunEvery30s()
{
  while (true)
    await Every30s();
}

await Task.WhenAll(RunEvery10s(), RunEvery30s());

answered Jun 24 '19 at 12:13

Stephen Cleary

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1

When and how will this program terminate? – Joelius Jun 24 '19 at 12:16
1

It will exit when all foreground threads have exited, just like any other program. – Stephen Cleary Jun 24 '19 at 12:19
But the call to `Task.WhenAll` won't ever return right? None of the passed in tasks will ever complete so the `Task.WhenAll` call won't return either or am I missing something? – Joelius Jun 24 '19 at 12:23
The `WhenAll` is a way to observe exceptions. If either of the operations fail, their exception will end up being thrown from `await Task.WhenAll`. – Stephen Cleary Jun 24 '19 at 12:26
Fair enough but this assumes that both methods implement some sort of cancellation by using an exception. I guess I'm missing some sort of handling the cancellation on the callers end, that's why I was irritated. – Joelius Jun 24 '19 at 12:31
I was referring to any kind of exception. If no exception is raised, these loops won't prevent the process from exiting. I agree cancellation would be a good thing to add, but as it wasn't in the op's original code, I didn't add it here, either. – Stephen Cleary Jun 24 '19 at 12:33
1

Thank you! I tried that before but without await methodname(); Works fine. I already implemented cancellation exceptions; the code was just a simplified example. – Pietr Podschinski Jun 24 '19 at 12:40
*"If either of the operations fail, their exception will end up being thrown from `await Task.WhenAll`"* -- I think that a failure of one operation alone will not get propagated by the `await Task.WhenAll`. Both operations must complete one way or another, for the `await Task.WhenAll` to complete. – Theodor Zoulias Mar 30 '22 at 01:40

How to run multiple methods asynchron and in parallel with delay

1 Answers1