storing the task completion time of every task using Task factory

Question

I am using task factory to spawn parallel threads and my code is as below. I have the requirement to print the completion time of every thread but dont know how to check for every thread. Currently my code is waiting for all tasks to finish and then calculating the time.

stp1.Start();
        for (int i = 0; i < tsk.Length; i++)
        {
            tsk[i] = Task.Factory.StartNew((object obj) =>
            {
                resp = http.SynchronousRequest(web, 443, true, req);
            }, i);
        }
        try
        {
            Task.WaitAll(tsk);
        }
        stp1.Stop();

Answer 1

You can add a continuation to the Task. Continuation is a method which will be called when the given Task is completed.

for (int i = 0; i < tsk.Length; i++)
{
    tsk[i] = Task.Factory.StartNew((object obj) =>
    {
        resp = http.SynchronousRequest(web, 443, true, req);
    }, i);

    tsk[i].ContinueWith(antecedent=> 
    {
        //antecedent is completed
        //Do whatever here                
    });
}

If you need to time individual tasks, you need one stopwatch per Task. You can start the StopWatch inside StartNew and stop it in ContinueWith.

If this is your actual code, you can simply time the synchronous operation which you invoke (http.SynchronousRequest in this case). For example following code is enough.

for (int i = 0; i < tsk.Length; i++)
{
    tsk[i] = Task.Factory.StartNew((object obj) =>
    {
        StopWatch watch = StopWatch.StartNew();

        resp = http.SynchronousRequest(web, 443, true, req);

        watch.Stop();
        Console.WriteLine(watch.Elapsed);
    }, i);
}

Btw, Network operations are inherently asynchronous; There will be asynchronous API available, you can use it instead of wrapping a synchronous web request in Task. For instance, maybe HttpClient.SendAsync.

Answer 2

I'd start off by saying that you don't need threadpool threads to execute async IO bound operations. Instead of using Task.Factory.StartNew, use naturally asynchronous API's such as the one HttpClient offers, for example.

Then, i'd say you can use Task.WhenAny to wait on the completion of each of your tasks as they finish:

// Note I'm assuming some kind of async implementation which returns a Task<T>
var tasks = tsk.Select(req => http.AsyncRequest(web, 443, true, req));

while (tasks.Count > 0)
{
    var finishedTask = await Task.WhenAny(tasks);
    // Do something with finishedTask

    tasks.Remove(finishedTask);
}