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
class MyClass {
int x;
void Foo( ) {
# if DEBUG
Console.WriteLine("Testing: x = {0}", x);
# endif
...
}In this class, the statement in Foo is compiled
conditionally, dependent upon the presence of the user-selected
DEBUG symbol. If you remove the
DEBUG symbol, the statement isn’t compiled.
Preprocessor symbols can be defined within a source file as just
shown, and they can be passed to the compiler with the
/define
:
symbol
command-line option. All preprocessor symbols are implicitly true, so
the previous #define statement is effectively
identical to:
#define DEBUG = true
The #error and #warning symbols
prevent accidental misuse of conditional directives by making the
compiler generate a warning or error given an undesirable set of
compilation symbols.
The C# language supports the preprocessor directives shown in Table 2.4.
Table 2-4. Preprocessor Directives