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.
With the #if
and #elif
directives, you can use the ||, &&, and
! operators to perform or, and, and not operations
on multiple symbols. The following directive instructs the compiler to include the code that
follows if the TESTMODE
symbol is defined and the
DEBUG
symbol is not defined:
#if TESTMODE && !DEBUG ...
Bear in mind, however, that you’re not building an ordinary C# expression, and the symbols upon which you operate have absolutely no connection to variables—static or otherwise.
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.
Table 1-15 lists all preprocessor directives and their actions.
Table 1-15. Preprocessor directives
Preprocessor directive |
Action |
---|---|
|
Defines |
|
Undefines |
|
Conditionally compiles code. |
|
Executes code to subsequent |
|
Combines |
|
Ends conditional directives. |
|
|
|
|
|
|
|
Marks the beginning of an outline. |
|
Ends an outline region. |
An attribute decorated with the Conditional
attribute will be compiled only if a given preprocessor
symbol is present, e.g.:
// file1.cs#define DEBUG
using System; using System.Diagnostics;[Conditional("DEBUG")]
public class TestAttribute : Attribute {} // file2.cs#define DEBUG
[Test] class Foo { [Test] string s; }
The compiler will not incorporate the [Test]
attributes if the DEBUG symbol is in scope for file2.cs.
The compiler generates a warning when it spots something in your code that seems unintentional. Unlike errors, warnings don’t ordinarily prevent your application from compiling.
Compiler warnings can be extremely valuable in spotting bugs. Their usefulness, however, is undermined when you get an excessive number of them. In a large application, maintaining a good signal-to-noise ratio is essential if the “real” warnings are to get noticed.
To this effect, the compiler allows you to selectively suppress warnings with the
#pragma warning
directive. In this example, we
instruct the compiler not to warn us about the field Message
not being used:
public class Foo { static void Main() { }#pragma warning disable 414
static string Message = "Hello";#pragma warning restore 414
}
Omitting the number in the #pragma warning
directive disables or restores all warning codes. If you are thorough in applying this
directive, you can compile with the /warnaserror
switch— this tells the compiler to treat any residual warnings as errors.