The preprocessor directives give instruction to the compiler to preprocess the information before actual compilation starts. All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a line. Preprocessor directives are not statements, so they do not end with a semicolon (;). C# compiler does not have a separate preprocessor; however, the directives are processed as if there was one. In C# the preprocessor directives are used to help in conditional compilation. Unlike C and C++ directives, they are not used to create macros. A preprocessor directive must be the only instruction on a line.

Preprocessor Directives in C#

The following table lists the preprocessor directives available in C# −
Sr.No. Preprocessor Directive & Description
1 #define It defines a sequence of characters, called symbol.
2 #undef It allows you to undefine a symbol.
3 #if It allows testing a symbol or symbols to see if they evaluate to true.
4 #else It allows to create a compound conditional directive, along with #if.
5 #elif It allows creating a compound conditional directive.
6 #endif Specifies the end of a conditional directive.
7 #line It lets you modify the compiler's line number and (optionally) the file name output for errors and warnings.
8 #error It allows generating an error from a specific location in your code.
9 #warning It allows generating a level one warning from a specific location in your code.
10 #region It lets you specify a block of code that you can expand or collapse when using the outlining feature of the Visual Studio Code Editor.
11 #endregion It marks the end of a #region block.

The #define Preprocessor

The #define preprocessor directive creates symbolic constants. #define lets you define a symbol such that, by using the symbol as the expression passed to the #if directive, the expression evaluates to true. Its syntax is as follows − 
#define symbol
The following program illustrates this − 
#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 useful for testing a symbol or symbols to check if they evaluate to true. If they do evaluate to true, the compiler evaluates all the code between the #if and the next directive. Syntax for conditional directive is − 
#if symbol [operator symbol]...
Where, symbol is the name of the symbol you want to test. You can also use true and false or prepend the symbol with the negation operator. The operator symbol is the operator used for evaluating the symbol. Operators could be either of the following −
  • == (equality)
  • != (inequality)
  • && (and)
  • || (or)
You can also group symbols and operators with parentheses. Conditional directives are used for compiling code for a debug build or when compiling for a specific configuration. A conditional directive beginning with a #if directive must explicitly be terminated with a #endif directive. The following program demonstrates use of conditional directives − 
#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