Differences between multithreading and multitasking in C# [duplicate]

Question

Possible Duplicate:
What is the difference between task and thread?

I understand the title itself may appear to be a duplicate question but I've really read all the previous posts related to this topic and still don't quite understand the program behavior.

I'm currently writing a small program that checks around 1,000 E-mail accounts. Undoubtedly I feel multithreading or multitasking is the right approach since each thread / task is not computationally expensive but the duration of each thread relies heavily on network I/O.

I think that under such a scenario, it would also be reasonable to set the number of threads / tasks at a number that is much larger than the number of cores. (four for i5-750). Therefore I've set the number of threads or tasks at 100.

The code snippet written using Tasks:

        const int taskCount = 100;
        var tasks = new Task[taskCount];
        var loopVal = (int) Math.Ceiling(1.0*EmailAddress.Count/taskCount);

        for (int i = 0; i < taskCount; i++)
        {
            var objContainer = new AutoCheck(i*loopVal, i*loopVal + loopVal);
            tasks[i] = new Task(objContainer.CheckMail);
            tasks[i].Start();
        }
        Task.WaitAll(tasks);

The same code snippet written using Threads:

        const int threadCount = 100;
        var threads = new Thread[threadCount];
        var loopVal = (int)Math.Ceiling(1.0 * EmailAddress.Count / threadCount);

        for (int i = 0; i < threadCount; i++)
        {
            var objContainer = new AutoCheck(i * loopVal, i * loopVal + loopVal);
            threads[i] = new Thread(objContainer.CheckMail);
            threads[i].Start();
        }
        foreach (Thread t in threads)
            t.Join();
        runningTime.Stop();
        Console.WriteLine(runningTime.Elapsed);

So what are the essential differences between these two?

Answer 1

Tasks do not necessarily correspond to threads. They will be scheduled by the task library to threadpool threads in a much more efficient manner than your thread code.

Threads are fairly expensive to create. Tasks will queue up and reuse threads as they become available, so when a thread is waiting for network IO, it can actually be reused to execute another task. Threads that sit idle are wasted resources. You can only execute the number of threads that corresponds to your processor core count (simultaneously), so 100 threads means context switches on all your cores at least 25 times each.

If using tasks, just queue up all 1000 email processing tasks instead of batching them up and let it rip. The task library will handle how many threads to run it on.