Why “lock” block not work?

Question

public class ThreadInteroperateWithLock
{
    private int m_count;
    private object m_synLock;
    public ThreadInteroperateWithLock() 
    {
        m_count = 0;
        m_synLock = new object();
    }

    public int Count { get { return m_count; } }

    public void Add() 
    {
        //just simulate some work
        int temp=0;
        for (int i = 0; i < 10000; i++)
        {
            temp++;
        }

        //really job
        lock (m_synLock)
        {
            m_count++;
        }
    }
}

This code is in a console application:

ThreadInteroperateWithLock ope = new ThreadInteroperateWithLock();
Thread[] threadArray = new Thread[100];
for (int i = 0; i < 100; i++)
{
    Thread thread = new Thread(new ThreadStart(ope.Add));
    thread.IsBackground = false;
    threadArray[i] = thread;
}
for (int i = 0; i < 100; i++)
{
    threadArray[i].Start();
}
Console.WriteLine(ope.Count);
Console.ReadKey();

Sometime it prints '99' and sometime '100', regardless of whether the lock{...} block exists or not. Is there any wrong in my code?

Answer 1

The problem here would be that you're kicking off the threads and they're not entirely finished by the time of your call to write output to the console.

The loop starts off i threads, we'll imagine these as bees going of collecting as work, not all going to the same food source so some take longer to return than others; then, back at the hive, we suddenly say: "Hey, bees, I need a headcount!", "...1, 2, 3..., only three?" No, some i-3 are still scrounging around outside!

So, the idea is that we must have an indicator of when work is completed, or a homing signal of sorts to get all bees back to the hive for the headcount. This can be done with Join , or manual state checking (which essentially has you holding out until the last sojourner returns.)

Answer 2

You're not waiting for the threads to finish. It'e entirely a matter of luck how many have done their work before you output the count.

Add

        for (int i = 0; i < 100; i++)
        {
            threadArray[i].Join();

        }

before the WriteLine , and you always get 100 .

Answer 3

You should protect your getter public int Count { get { return m_count; } } public int Count { get { return m_count; } } with a lock to, else a thread B could be reading that value, while another thread A is updating count in your Add method, this can cause you to get an inconsistent view of your data.

Answer 4

It's a classic race condition , and I consider you lucky to get anywhere near 100 by the time your Console.WriteLine is invoked.

for (int i = 0; i < 100; i++)
{
    threadArray[i].Start();
}
//absolutely no guarantee that **ANY** threads have completed before next line
Console.WriteLine(ope.Count);

Consider using a CountdownEvent or similar to do a bit of synchronization.

Answer 5

It is not deterministic when your call to Count happens. It may happen before any thread has finished, or after all threads have finished or even somewhere in the middle. If you want to wait until all thread have finished, you should join them.

Answer 6

不确定这是你正在寻找的答案，但要以线程安全的方式增加计数器，你应该使用Interlocked.Increment ^docs

Answer 7

When you print the count you have no idea of whether or not the threads have completed or not. A "correct" output in your code could be any number between 0 and 100.

To be certain to get 100 as output, you mustn't get the value of Count until all threads have completed. You can do that by calling Thread.Join on all threads or (even better) use something like Parallel.For to start the threads:

Parallel.For(0, 100, idx => ope.Add);
// When we reach this line, we know all threads have completed so we can 
// safely get the count
var count = ope.Count;