为什么并行任务不运行？

Question

I have this code, it is the skeleton of larger functionality stripped down to prove the problem:

        var tasks = Enumerable.Range(0, 10)
            .Select(laneNo => Task.Run(() => Console.WriteLine($"Starting generator for lane {laneNo}")));

        for(int r=0; ;++r)
        {
            Task.Delay(TimeSpan.FromSeconds(3)).Wait();

            Console.WriteLine($"Iteration {r} at {DateTime.Now}");
        }

I never see "Starting generator" printed to Console but I do see the iteration fire every 3 seconds - something is causing those tasks not to progress (in the real code they run for a significant period but removing that doesn't affect the problem).

Why are the first bunch of Tasks not running? My theory is it's related to Task.Delay ?

Answer 1

Your linq-statment is never materialized. Linq-operators like Select, Where, OrderBy, etc work as building blocks that you chain together but they are not executed until you run it through a foreach or use operators which do not return enumerables, like ToArray, ToList, First, Last etc.

If you call ToList at the end you should see all of the tasks executing but if you only call First you should see only a single one because the iteration of your original Range will then terminate after first element.

Answer 2

LINQ Select has deferred execution; it simply defines an iterator, so your Task s are not being generated.

You could make use of Task.WhenAll(IEnumerable<Task>) , which will iterate and await each Task , generating new Task that completes once all the provided tasks have also completed:

var tasks = Enumerable.Range(0, 10)
        .Select(laneNo => Task.Run(() => Console.WriteLine($"Starting generator for lane {laneNo}")));

await Task.WhenAll(tasks);