Preprocessor Directives

What Are Preprocessor Directives?
General Rules
The #define and #undef Directives
Conditional Compilation
The Conditional Compilation Constructs
Diagnostic Directives
Line Number Directives
Region Directives
The #pragma warning Directive

What Are Preprocessor Directives?
The source code specifies the definition of a program. The preprocessor directives instruct the compiler how to treat the source code. For example, under certain conditions, you might want the compiler to ignore portions of the code, and under other conditions, you might want that code compiled. The preprocessor directives give you those options and several others.
In C and C++ there is an actual preprocessor phase, in which the preprocessor goes through the source code and prepares an output stream of text to be processed by the subsequent compilation phase. In C# there is no actual preprocessor. The “preprocessor” directives are handled by the compiler. The term, however, remains.

General Rules
Some of the most important syntactic rules for preprocessor directives are the following:
• Preprocessor directives must be on lines separate from C# code.
• Unlike C# statements, preprocessor directives are not terminated with a semicolon.
• Every line containing a preprocessor directive must start with the # character.
– There can be space before the # character.
– There can be space between the # character and the directive.
• End-of-line comments are allowed.
• Delimited comments are not allowed in a preprocessor directive line.

Here are some examples illustrating the rules: The preprocessor directives are listed in Table 23-1. The #define and #undef Directives
A compilation symbol is an identifier that has only two possible states. It is either defined or undefined. A compilation symbol has the following characteristics:
• It can be any identifier except true or false. This includes C# keywords, and identifiers declared in your C# code—both of which are fine.
• It has no value. Unlike in C and C++, it does not represent a string.
As shown in Table 23-1
• The #define directive declares a compilation symbol.
• The #undef directive undefines a compilation symbol.

#define PremiumVersion
#define EconomyVersion
...
#undef PremiumVersion

The #define and #undef directives can be used only at the top of a source file, before any C# code is listed. After the C# code has started, the #define and #undef directives can no longer be used. The scope of a compilation symbol is limited to a single source file. Redefining a symbol that is already defined is perfectly fine—as long as it’s before any C# code, of course. Conditional Compilation
Conditional compilation allows you to mark a section of source code to be either compiled or skipped, depending on whether a particular compilation symbol is defined.
There are four directives for specifying conditional compilation:
• #if
• #else
• #elif
• #endif
A condition is a simple expression that returns either true or false.
• A condition can consist of a single compilation symbol, or an expression of symbols and operators, as summarized in Table 23-2. Sub-expressions can be grouped with parentheses.
• The literals true and false can also be used in conditional expressions.
Table 23-2. Conditions Used in the #if and #elif Directives The following are examples of conditional compilation conditions: The Conditional Compilation Constructs
The #if and #endif directives are the matching demarcations of a conditional compilation construct. Whenever there is an #if directive, there must also be a matching #endif.
The #if and #if...#else constructs are illustrated in Figure 23-1.
• If the condition in the #if construct evaluates to true, the code section following it is compiled. Otherwise, it is skipped.
• In the #if...#else construct, if the condition evaluates to true, CodeSection1 is compiled. Otherwise, CodeSection2 is compiled. Figure 23-1. The #if and #else constructs

For example, the following code illustrates a simple #if...#else construct. If the symbol RightHanded is defined, the code between the #if and the #else will be compiled. Otherwise, the code between the #else and the #endif will be compiled. The #if...#elif and #if...#elif...#else constructs are illustrated in Figure 23-2.
• In the #if...#elif construct, if Cond1 evaluates to true, CodeSection1 is compiled, and compilation continues after the #endif.
– Otherwise, if Cond2 evaluates to true, CodeSection2 is compiled, and compilation continues after the #endif.
– This continues until either a condition evaluates to true, or all the conditions have returned false. If that is the case, none of the code sections in the construct are compiled, and compilation continues after the #endif.
• The #if...#elif...#else construct works the same way, except that if no condition is true, then the code section after the #else is then compiled, and compilation continues after the #endif. Figure 23-2. The #elif construct

The following code demonstrates the #if...#elif...#else construct. The string containing the description of the version of the program is set to various values, depending on which compilation symbol is defined. Diagnostic Directives
Diagnostic directives produce user-defined compile-time warning and error messages.
The following is the syntax of the diagnostic directives. The messages are strings, but notice that unlike normal C# strings, they do not have to be enclosed in quotation marks.
#warning Message
#error Message

When the compiler reaches a diagnostic directive, it writes out the associated message. The diagnostic directive messages are listed by the compiler along with any compilergenerated warning and error messages.
For example, the following code shows an #error directive and a #warning directive.
• The #error directive is inside an #if construct, so that it will be generated only if the conditions on the #if directive are met.
• The #warning directive is a reminder to the programmer to come back and clean up a section of code.
#define RightHanded
#define LeftHanded
#if RightHanded && LeftHanded
#error Can't build for both RightHanded and LeftHanded
#endif
#warning Remember to come back and clean up this code!

Line Number Directives
Line number directives can do several things, including the following:
• Change the apparent line numbers reported by the compiler’s warning and error messages
• Change the apparent filename of the source file being compiled
• Hide a sequence of lines from the interactive debugger
The syntax for the #line directives is the following: The #line directive with an integer parameter causes the compiler to consider that value to be the line number of the following line of code. Numbering of the subsequent lines continues, based on that line number.
• To change the apparent filename, use the filename, inside double quotes, as the parameter. The double quotes are required.
• To return to true line numbering and the true filename, use default as the parameter.
• To hide a segment of code from the step-through-code feature of the interactive debugger, use hidden as the parameter. To stop hiding, use the directive with no parameter. This feature has, so far, mostly been used in ASP.NET for hiding compiler-generated code.
The following code shows examples of the line number directives: Region Directives
The region directive allows you to mark, and optionally name, a section of code. The #region directive
• Is placed on the line above the section of code you want to mark
• Can have an optional string of text following it on the line, which serves as its name
• Must be terminated by an #endregion directive, farther down in the code
Although region directives are ignored by the compiler, they can be used by source code tools. Visual Studio, for example, allows you to easily hide or display regions.
As an example, the following code has a region called Constructors, which encloses the two constructors of class MyClass. In Visual Studio, you could collapse this region to a single line when you didn’t want to see it in the code, and then expand it again when you needed to work on it or add another constructor. Regions can be nested, as shown in Figure 23-3. Figure 23-3. Nested regions

The #pragma warning Directive
The #pragma warning directive allows you to turn off warning messages and to turn them back on.
• To turn off warning messages, use the disable form with a comma-separated list of warning numbers you want to turn off.
• To turn warning messages back on, use the restore form with a list of the warning numbers you want to turn back on.
For example, the following code turns off two warning messages: 618 and 414. Farther down in the code, it turns on messages for 618 but leaves the messages for 414 turned off. If you use either form without a warning number list, the command then applies to all warnings. For example, the following code turns off, and then restores, all warning messages.