This is not the same thing, what you do here is not a pre-processor error, the compiler will believe MY_MACRO is an undeclared symbol.

I am talking about :

#if NOT_DEFINED // if NOT_DEFINED is undefined, then the default behavior is to consider it 0
we are not in :(
#endif

#if !ANOTHER 
we are in :)
#endif

#if YET_ANOTHER + 2 == 2
we are in :) but did we want it !
#endif

Ok. But I used an undefined symbol, concatenated it with a "b" in some other macro and used in some other macro to generate a cbuffer declaration which must have resulted in something like:

cbuffer Widget : register(b) {

...

}