Preprocessor Directives
Preprocessor directives supply the compiler with additional information about regions of code. The most common preprocessor directives are the conditional directives, which provide a way to include or exclude regions of code from compilation. For example:
#define DEBUG using System; class MyClass { static int x = 5; static void Main( ) { # if DEBUG Console.WriteLine("Testing: x = {0}", x); # endif } }
In this class, the statement in Foo
is compiled as
conditionally dependent upon the presence of the
DEBUG
symbol. If we remove the
DEBUG
symbol, the statement is not compiled.
Preprocessor symbols can be defined within a source file (as we have
done) and can be passed to the compiler with the
/define
: symbol command-line option.
The #error
and #warning
symbols
prevent accidental misuse of conditional directives by making the
compiler generate a warning or error when given an undesirable set of
compilation symbols. See Table 1-5 for a list
of preprocessor directives and their actions.
Table 1-5. Preprocessor directives
Preprocessor directive |
Action |
---|---|
|
Defines |
|
Undefines |
|
|
|
Executes code to subsequent |
|
Combines |
|
Ends conditional directives |
|
|
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