040 C# Preprocessor Directives

Preprocessor directive
Preprocessor directive gives instruction compiler , before the actual compilation starts.  Preprocessor directives begin with #, but do not ends with semicolon (;) . C and C++  have separate processor, which create macros . C# do not have any separate Preprocessor.  C# considers Preprocessor directive as  instruction.

A simple program with Preprocessor directive

Example  

#define mySysmbol

using System;

public class Program

{

              public static void Main()

               {

                   #if (mySysmbol)

                      Console.WriteLine("mySysmbol is defined");

                  #endif

                 Console.ReadKey();

            }

}

 Output :
mySysmbol is defined

Here is the most common used Preprocessor directive.

directive	What is ?
#define	define symbol Preprocessor. The symbol maybe sequence of character
#undefine	undefine the symbol which was defined by the #define.
#if	test or check particular symbol or symbol with compound statement with operator return true or false. If true is return , some statement is executed
#else	if #if returns false , this code block is executed with some statement
#elif	checking of compound conditional statement on define by the #define
#endif	close or end code block started from #if directives
#line	compiler line number can be customize
#error	customized error , is generated from desired location
#warning	customize warning is generated from desired location
#region	specify an area code blocks. Expand and collapse functionalities is by defauly in .Net framework
#endregion	end code block area define by #define

A Preprocessor directive is defined at the top of a program as it is defined before the compilation of program . Preprocessor directive symbols  and same name variable can be declared, both do not conflict each other.

A simple program with Preprocessor directive , directive symbol and variable are same name.

Example  

#define mySysmbol

using System;

public class Program

{

          public static void Main()

         {

                    string mySysmbol = string.Empty;

                    mySysmbol = "I Am Learning C#";

                   #if (mySysmbol)

                               Console.WriteLine("mySysmbol is defined");

                  #endif

                  Console.ReadKey();

           }

}

Output :
mySysmbol is defined

If you are writing check statement (if) , you can use operator like =, == ,!=,&&,||,! . #region  is generally written when long term code is written , code is  separated  with different part of logic.

A simple program with Preprocessor directive , with operator.

Example

#define mySysmbol1

#define mySysmbol2

using System;

            public class Program

            {

                          public static void Main()

                         {

                                #if (mySysmbol1 && mySysmbol2)

                                {

                                           Console.WriteLine("mySysmbol1 and mySysmbol2 defined");

                                }

                               #endif

                               Console.ReadKey();

                    }

}

Output :
mySysmbol1 and mySysmbol2 defined