#define (C# Reference)
#define lets you define a symbol, so that by using the symbol as the expression passed to the #if directive, the expression will evaluate to true. For example:
You can define a symbol, but you cannot assign a value to a symbol. The #define directive must appear in the file before you use any instructions that are not also directives.
A symbol that you define with /define or with #define does not conflict with a variable of the same name. That is, a variable name should not be passed to a preprocessor directive and a symbol can only be evaluated by a preprocessor directive.
The scope of a symbol created with #define is the file in which it was defined.
See #if for an example of how to use #define.