Strange preprocessing

Something strange happens.

I have in my code in a local function

#ifdef abc
.
.
.
#endif

when I write on the top of the file

#define abc

it acts as if it isn't defined. only when I define

#define abc

in the local function itself it reads the code in the ifdef.

This is strange because I thought that the compiler just goes over the whole file without details of functions to change the # preprocessing orders. can't understand what's happening. I am using visual-studio 2005.