using interlocked usage

Question

I am trying to write a multithreaded program where each thread would use a counter and then increment it.

for example:

lock(this)
{
   counter++;
   Console.WriteLine(counter); 
}

i know that for incrementation i can use:

System.Threading.Interlocked.Increment(counter);

but what about locking for both incrementing and doing something with the counter?

Thanks!

Answer 1

Doing this is OK:

Thread A:

var incremented_counter = Interlocked.Increment(ref counter);
Console.WriteLine(incremented_counter);

Thread B:

Interlocked.Increment(ref counter);

---

And doing this is OK:

Thread A:

lock (the_lock) {
   ++counter;
   Console.WriteLine(counter); 
}

Thread B:

lock (the_lock) {
   ++counter;
}

---

Doing this is OK but redundant:

Thread A:

lock (the_lock) {
    var incremented_counter = Interlocked.Increment(ref counter);
    Console.WriteLine(incremented_counter);
}

Thread B:

lock (the_lock) {
    Interlocked.Increment(ref counter);
}

---

But doing this is not OK:

Thread A:

Interlocked.Increment(ref counter);
Console.WriteLine(counter);

Thread B:

Interlocked.Increment(ref counter);

---

Nor is it doing this:

Thread A:

lock (the_lock) {
   ++counter;
   Console.WriteLine(counter); 
}

Thread B:

Interlocked.Increment(ref counter);

---

Nor is it doing this:

Thread A:

var incremented_counter = Interlocked.Increment(ref counter);
Console.WriteLine(incremented_counter);

Thread B:

lock (the_lock) {
   ++counter;
}

(BTW, don't use lock on this.)

Answer 2

All of the Interlock functions return a copy of the value after modification, used that returned value during your thread.

var localCounter = System.Threading.Interlock.Increment(counter);