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.

Some of the most important syntactic rules for preprocessor directives are the following:

Here are some examples illustrating the rules:

The preprocessor directives are listed in Table 23-1.

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:

As shown in Table 23-1

#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.

using System;                        // First line of C# code
   #define PremiumVersion               // Error

   namespace Eagle
   {
      #define PremiumVersion            // Error
      ...

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.

#define AValue
   #define BValue

   #define AValue                       // Redefinition is fine.

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:

A condition is a simple expression that returns either true or false.

The following are examples of conditional compilation conditions:

Expression

   #if !DemoVersion
      ...
   #endif               Expression

   #if (LeftHanded && OemVersion) || FullVersion
      ...
   #endif

   #if true   // The following code segment will always be compiled.
      ...
   #endif

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.

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.

...
   #if RightHanded
      // Code implementing right-handed functionality
      ...
   #else
      // Code implementing left-handed functionality
      ...
   #endif

The #if...#elif and #if...#elif...#else constructs are illustrated in Figure 23-2.

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.

#define DemoVersionWithoutTimeLimit
         ...
      const int intExpireLength = 30;
      string strVersionDesc = null;
      int    intExpireCount = 0;

   #if   DemoVersionWithTimeLimit
      intExpireCount = intExpireLength;
      strVersionDesc = "This version of Supergame Plus will expire in 30 days";

   #elif DemoVersionWithoutTimeLimit
      strVersionDesc = "Demo Version of Supergame Plus";

   #elif OEMVersion
      strVersionDesc = " Supergame Plus, distributed under license";

   #else
      strVersionDesc = "The original Supergame Plus!!";

   #endif

      Console.WriteLine( strVersionDesc );
         ...

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 compiler generated warning and error messages.

For example, the following code shows an #error directive and a #warning directive.

#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 can do several things, including the following:

The syntax for the #line directives is the following:

#line integer           // Sets line number of next line to value of integer
   #line "filename"        // Sets the apparent filename
   #line default           // Restores real line number and filename

   #line hidden            // Hides the following code from stepping debugger
   #line                   // Stops hiding from debugger

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.

The following code shows examples of the line number directives:

#line 226
         x = y + z; // Now considered by the compiler to be line 226
         ...

   #line 330 "SourceFile.cs" // Changes the reported line number and filename
         var1 = var2 + var3;
         ...

   #line default              // Restores true line numbers and filename

The region directive allows you to mark, and optionally name, a section of code. The #region directive

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.

#region Constructors
      MyClass()
      { ... }

      MyClass(string s)
      { ... }
   #endregion

Regions can be nested, as shown in Figure 23-3.

Figure 23-3. Nested regions

The #pragma warning directive allows you to turn off warning messages and to turn them 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.

#pragma warning disable
...        All warning messages are turned off in this section of code.

#pragma warning restore
...        All warning messages are turned back on in this section of code.