Guide to the Language

CHAPTER 25
Preprocessors
C# includes a set of preprocessor directives that are mainly used for 
conditional compilation. Although the C# compiler does not have a 
separate preprocessor, as C and C++ compilers, the directives shown here 
are processed as if there were one. That is, they appear to be processed 
before the actual compilation takes place.
Directive
Description
#if
#elif
#else
#endif
If
Else if
Else
End if
#define
#undef
Symbol define
Symbol undefine
#error
#warning
#line
Generate error
Generate warning
Set line number
#region
#endregion
Mark section start
Mark section end

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 Preprocessor Syntax
The preprocessor directives are easily distinguished from normal 
programming code in that they start with a hash sign (#). They must always 
occupy a line that is separate from anything else, except for single-line 
comments. Whitespace may optionally be included before and after the 
hash mark.
#line 1 // set line number
 Conditional Compilation Symbols
A conditional compilation symbol is created using the #define directive 
followed by the symbol’s name. When a symbol is defined, it will then 
cause a conditional expression using that condition to be evaluated as 
true. The symbol will remain defined only within the current source file, 
starting from the line where the symbol is created.
#define MySymbol
The #undef (undefine) directive can disable a previously defined 
symbol.
#undef MySymbol
 Conditional Compilation
The #if and #endif directives specify a section of code that will be 
included or excluded based on a given condition. Most often, this 
condition will be a conditional compilation symbol.
#if MySymbol
// ...
#endif
ChaptEr 25 prEproCESSorS

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Just as with the C# if statement, the #if directive can optionally 
include any number of #elif (else if) directives and one final #else 
directive. Conditional directives may also be nested within another 
conditional section. In longer conditionals, it is good practice to add 
comments to the #endif directives to help keep track of which #if 
directive they correspond to.
#if Professional
// ...
#elif Advanced || Enterprise
// ...
#else
#if Debug
// ...
#endif // Debug
#endif // Professional