In C#, what does “atomic” mean?

Question

I read this in the book C# 6.0 and the .NET 4.6 framework:

“assignments and simple arithmetic operations are not atomic”.

So, what d

Answer 1

Atomic operations are ones that cannot be interrupted partway through, such as by threading. Take for instance the statement

_value++;

If you have two threads executing this code at once with a starting value of 0, you may have the following

• Thread A reads _value, 0
• Thread A adds 1, 1
• Thread B reads _value, 0
• Thread B adds 1, 1
• Thread A assigns to _value, 1
• Thread B assigns to _value, 1

so now, even though we've called an increment twice, the final value in _value is 1, not the expected 2. This is because increment operators are not atomic.

The function Interlocked.Increment, however, is atomic, so replacing the above code with

Interlocked.Increment(ref _value);

Would solve the given race condition.

EDIT: As a point of etymology, "atomic" usually means "indivisible" - the chemistry term we're familiar with is a misnomer held over from the belief that atoms were indivisible, only for later discoveries to break them down further into subatomic, quark, and quanta levels.