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 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 they can be passed to the compiler with the /define:
symbol
command-line option. All preprocessor symbols are implicitly true, so the #define
statement
in the previous example is effectively the same as the following:
#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
See Table 4-3 for a list of preprocessor directives and their actions.
Table 4-3. Preprocessor directives
Preprocessor directive |
Action |
---|---|
#define
symbol
|
Defines |
#undef
symbol
|
Undefines |
#if
symbol
[
operator symbol2
]
...
|
|
#else
|
Executes code to subsequent |
#elif
symbol
[
operator symbol2
]
|
Combines |
#endif
|
Ends conditional directives |
#warning
text
|
|
#error
text
|
|
#line
number
["
file
"]
|
|
#region
name
|
Marks the beginning of outline |
#end
region
|
Ends an outline region |
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