When to use 'volatile' or 'Thread.MemoryBarrier()' in threadsafe locking code? (C#)

Question

When should I use volatile/Thread.MemoryBarrier() for thread safety?

Answer 1

You use volatile/Thread.MemoryBarrier() when you want to access a variable across threads without locking.

Variables that are atomic, like an int for example, are always read and written whole at once. That means that you will never get half of the value before another thread changes it and the other half after it has changed. Because of that you can safely read and write the value in different threads without syncronising.

However, the compiler may optimize away some reads and writes, which you prevent with the volatile keyword. If you for example have a loop like this:

sum = 0;
foreach (int value in list) {
   sum += value;
}

The compiler may actually do the calculations in a processor register and only write the value to the sum variable after the loop. If you make the sum variable volatile, the compiler will generate code that reads and writes the variable for every change, so that it's value is up to date throughout the loop.