This article was previously published in my blog, Just Like a Magic.
Actually, I was having a problem synchronizing threads calling a function. If we could regenerate the bug, we would end up with code like this:
static void Main()
{
Thread[] arr = new
Thread[10];
for (int i = 0; i < 10; i++)
{
arr[i] = new
Thread(ThreadProc);
arr[i].IsBackground = true;
arr[i].Start(new object());
}
foreach
(Thread t in arr)
t.Join();
}
private static void ThreadProc(object
obj)
{
lock (obj)
{
int i =
0;
while
(i < 10)
{
Thread.Sleep(1);
Console.WriteLine("Thread #{0},\t{1}",
Thread.CurrentThread.ManagedThreadId,
i++);
}
}
}
And when we execute this code the results would be like this:
Thread #4, 2
Thread #3, 3
Thread #5, 1
Thread #7, 0
Thread #5, 2
Thread #6, 1
Thread #3, 4
Thread #4, 3
Thread #8, 1
Thread #9, 0
What is the problem with this code? It starts multiple threads and passes them a locking object and the object is locked successfully using the lock statement, so why threads overlap? Why the synchronization doesn't take effect?
Well, after a long period and after pushing a new thread in the MSDN forums (we are all developers do make silly mistakes, aih? :P), I come up with the solution and discovered the drawback of the code.
The problem was that each time we start off a thread we pass it a new locking object different from the others:
arr[i].Start(new object());
Therefore, every thread is locking on its own objects, so no thread synchronization take effect.
The solution is very easy, you should lock on a shared object; an object that is shared between all your threads accessing this block of code. Read more about thread synchronization here.
For example, we should change our code to the following:
private static object _lockThis = new object();
static void Main()
{
Thread[] arr = new
Thread[10];
for (int i = 0; i < 10; i++)
{
arr[i] = new
Thread(ThreadProc);
arr[i].IsBackground = true;
arr[i].Start();
}
foreach
(Thread t in arr)
t.Join();
}
private static void ThreadProc(object
obj)
{
lock
(_lockThis)
{
int i =
0;
while
(i < 10)
{
Thread.Sleep(1);
Console.WriteLine("Thread #{0},\t{1}",
Thread.CurrentThread.ManagedThreadId, i++);
}
}
}
Finally, this was one of the bad practices and mistakes me (and many others too) fall in. Honestly, I would like to start my Bad Practices series :">. I would write about problems I came across and solutions I found for them. In addition, I'm thinking of starting the Questions series. I got a huge number of questions every week, if we could publish them I think it would be great for all.
Have a nice day!