How do I know if this C# method is thread safe?

Question

I'm working on creating a call back function for an ASP.NET cache item removal event.

The documentation says I should call a method on an object or calls I know will exist (will be in scope), such as a static method, but it said I need to ensure the static is thread safe.

Part 1: What are some examples of things I could do to make it un-thread safe?

Part 2: Does this mean that if I have

static int addOne(int someNumber){
    int foo = someNumber;
    return foo +1; 
}

and I call Class.addOne(5); and Class.addOne(6); simutaneously, Might I get 6 or 7 returned depending on who which invocation sets foo first? (i.e. a race condition)

Answer 1

That addOne function is indeed thread safe because it doesn't access any data that could be accessed by another thread. Local variables cannot be shared among threads because each thread gets its own stack. You do have to make sure, however, that the function parameters are value types and not reference types.

static void MyFunction(int x) { ... } // thread safe. The int is copied onto the local stack.

static void MyFunction(Object o) { ... } // Not thread safe. Since o is a reference type, it might be shared among multiple threads.