任务并行库 (TPL) 是否处理竞争条件

Question

I am trying to understand the benefits of Task Parallel library over using traditional multi threading and when I think about the below situation, I am stuck thinking does it handle race condition or do we need to handle this in our code?

Here is my code:

 int depdt = 0;    
 Parallel.For(1, 10, mem =>
        {

            for (int dep = 1; dep <= 10; dep++)
            {
                depdt = dep;
                Console.WriteLine("MEMBER: " + mem + " " + "Dependent: " + dep);
            }

            Console.WriteLine("Dep Value: " + depdt + " " + "mem: " + mem);
        });
        Console.ReadKey();

I ran it couple of times and I don't see any thread interfering/overwriting the the "depdt" variable. But I need to confirm this. (or) To make it thread safe should I manually create an instance of class and implement it like the below code to avoid race conditions

 int depdt = 0;
        Parallel.For(1, 10, mem =>
        {
              Worker worker = new Worker();
              worker.DoWork(mem);

        });
        Console.ReadKey();

  public class Worker
{
    public void DoWork(int mem)
    {

        int depdt = 0;
        for (int dep = 1; dep <= 10; dep++)
        {
            depdt = dep;
            Console.WriteLine("MEMBER: " + mem + " " + "Dependent: " + dep);
        }

        Console.WriteLine("Dep Value: " + depdt  +" "+ "mem: "+ mem);
    }
}

Response to @yms: I mean when using normal threads the varaible depdt becomes unreliable. Here is my code:

for (int mem = 1; mem <= 10; mem++)
        {
            var t= new Thread(state =>
            {
                for (int dep = 1; dep <= 10; dep++)
                {
                    depdt = dep;
                    Console.WriteLine("MEMBER: " + mem + " " + "Dependent: " + dep);
                }

                Console.WriteLine("Dep Value: " + depdt + " " + "mem: " + mem);
            });

            t.Start(string.Format("Thread{0}", mem));
        }
        Console.ReadKey();

Here is my output screen: Infact both mem and dep variables have become unreliable

Answer 1

If you expect your program to always write Dep Value: 10 , then yes, your program is subject to a race condition that may lead to other values being printed. To demonstrate the issue, just introduce a delay within the inner loop:

int depdt = 0;
Parallel.For(1, 10, mem =>
{
    for (int dep = 1; dep <= 10; dep++)
    {
        depdt = dep;
        Console.WriteLine("MEMBER: " + mem + " " + "Dependent: " + dep);
        Thread.Sleep(mem * 100);   // delay introduced here
    }
    Console.WriteLine("Dep Value: " + depdt + " " + "mem: " + mem);
});
Console.ReadKey();

The reason that your program appears to behave correctly is that the inner loop takes very little time to execute, probably completing within a single time quantum allocated to the thread.

To avoid the race condition, you just need to move the depdt declaration inside the anonymous function passed to Parallel.For . This would cause each thread to have its own variable, avoiding conflicts.

Parallel.For(1, 10, mem =>
{
    int depdt = 0;
    for (int dep = 1; dep <= 10; dep++)
    {
        depdt = dep;
        Console.WriteLine("MEMBER: " + mem + " " + "Dependent: " + dep);
    }
    Console.WriteLine("Dep Value: " + depdt + " " + "mem: " + mem);
});

Answer 2

No. Task parallel library does not handles the race conditions by default. You need to take care of synchronizing the access to the shared resources.