Is Interlock.CompareExchange atomic inside an if statement?

Question

I am assuming that the piece of code is threadsafe,

if ((Interlocked.CompareExchange(ref semaphore, 1, 0) == 0))
{
  //Do Stuff
  semaphore = 0;
}

However I am wondering why:

In my mind I see this as two operations,

1. First it compares the semaphore with the value, and replaces it if they are equal (Interlocked.CompareExchange(ref semaphore, 1, 0)

2. Then it compares the value it returns to 0, with the stamement {CompareExchange} == 0

Can not the semaphore bet set somewhere else, before the second comparison (2.) happens? And that the function gets entered when it should not?

Answer 1

Interlocked.CompareExchange is an atomic operation, resulting in a return value. Once that happens, the result of that method call cannot change unless you call Interlocked.CompareExchange again, even if semaphore changes. It doesn't matter that it is inside an if statement; you can depend on that return value to be stable.