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C# `Preprocessor Directives`

Preprocessor directives instruct the compiler to preprocess the information before actual compilation begins.

All preprocessor directives start with a # and are on a single line. Only whitespace characters can appear before a preprocessor directive. Preprocessor directives are not statements, so they do not end with a semicolon (;).

The C# compiler does not have a separate preprocessor; however, directives are processed as if there were one. In C#, preprocessor directives are used for conditional compilation. Unlike C and C++, they are not used to create macros. A preprocessor directive must be the only instruction on a line.

C# Preprocessor Directives List

The following table lists the preprocessor directives available in C#:

Preprocessor Directive	Description
#define	It is used to define a symbol.
#undef	It is used to undefine a symbol.
#if	It is used to test if a symbol is true.
#else	It is used to create a compound conditional directive, used with #if.
#elif	It is used to create a compound conditional directive.
#endif	Specifies the end of a conditional directive.
#line	It allows you to modify the compiler's line number and (optionally) the file name for errors and warnings.
#error	It allows generating an error from a specified location in the code.
#warning	It allows generating a level one warning from a specified location in the code.
#region	It allows you to specify an expandable or collapsible code block when using the outline feature of the Visual Studio Code Editor.
#endregion	It marks the end of a #region block.

#define Preprocessor

The #define preprocessor directive creates a symbolic constant.

define allows you to define a symbol, so that by using the symbol as the expression passed to the #if directive, the expression will return true. Its syntax is as follows:

#define symbol

The following program illustrates this:

Example

#define PI 
using System;
namespace PreprocessorDAppl
{
   class Program
   {
      static void Main(string[] args)
      {
         #if (PI)
            Console.WriteLine("PI is defined");
         #else
            Console.WriteLine("PI is not defined");
         #endif
         Console.ReadKey();
      }
   }
}

When the above code is compiled and executed, it produces the following result:

PI is defined

Conditional Directives

You can use the #if directive to create a conditional directive. Conditional directives are used to test if a symbol is true. If true, the compiler executes the code between #if and the next directive.

The syntax for conditional directives is:

#if symbol [operator symbol]...

Where symbol is the name of the symbol to test. You can also use true and false, or place a negation operator before the symbol.

Common operators include:

== (equal to)
!= (not equal to)
&& (and)
|| (or)

You can also group symbols and operators with parentheses. Conditional directives are used to compile code for debugging or specific configurations. A conditional directive starting with the #if directive must explicitly end with a #endif directive.

The following program demonstrates the use of conditional directives:

Example

#define DEBUG
#define VC_V10
using System;
public class TestClass
{
   public static void Main()
   {
      #if (DEBUG && !VC_V10)
         Console.WriteLine("DEBUG is defined");
      #elif (!DEBUG && VC_V10)
         Console.WriteLine("VC_V10 is defined");
      #elif (DEBUG && VC_V10)
         Console.WriteLine("DEBUG and VC_V10 are defined");
      #else
         Console.WriteLine("DEBUG and VC_V10 are not defined");
      #endif
      Console.ReadKey();
   }
}

When the above code is compiled and executed, it produces the following result:

DEBUG and VC_V10 are defined

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