C# 2008 Programmer's Reference

The Main() method is located in the Program.cs file. The program basically asks the user to enter a number and then sums up all the odd number from 1 to that number:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace TestDefine {
 class Program {
  static void Main(string[] args) {
   Console.Write('Please enter a number: ');
   int num = int.Parse(Console.ReadLine());
   int sum = 0;
   for (int i = 1; i <= num; i++) {
    //---sum up all odd numbers---
    if (i % 2 == 1) sum += i;
   }
   Console.WriteLine(
    'Sum of all odd numbers from 1 to {0} is {1}',
    num, sum);
   Console.ReadLine();
  }
 }
}

Suppose that you want to add some debugging statements to the program so that you can print out the intermediate results. The additional lines of code are highlighted:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace TestDefine {
 class Program {
  static void Main(string[] args) {
   Console.Write('Please enter a number: ');
   int num = int.Parse(Console.ReadLine());
   int sum = 0;
   for (int i = 1; i <= num; i++) {
    //---sum up all odd numbers---
    if (i % 2 == 1)
    {
     sum += i;
     Console.WriteLine('i={0}, sum={1}', i, sum);
    }
   }
   Console.WriteLine(
    'Sum of all odd numbers from 1 to {0} is {1}',
    num, sum);
   Console.ReadLine();
  }
 }
}

You do not want the debugging statements to be included in the production code so you first define a symbol (such as DEBUG) using the #define preprocessor directive and wrap the debugging statements with the #if and #endif preprocessor directives:

#define DEBUG
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace TestDefine {
 class Program {
  static void Main(string[] args) {
   Console.Write('Please enter a number: ');
   int num = int.Parse(Console.ReadLine());
   int sum = 0;
   for (int i = 1; i <= num; i++) {
    //---sum up all odd numbers---
    if (i % 2 == 1) {
     sum += i;
#if DEBUG
     Console.WriteLine('i={0}, sum={1}', i, sum);
#endif
    }
   }
   Console.WriteLine(
    'Sum of all odd numbers from 1 to {0} is {1}',
    num, sum);
   Console.ReadLine();
  }
 }
}

DEBUG is a common symbol that developers use to indicate debugging statements, which is why most books use it in examples. However, you can define any symbol you want using the #define preprocessor directive.

Before compilation, the preprocessor will evaluate the #if preprocessor directive to see if the DEBUG symbol has been defined. If it has, the statement(s) wrapped within the #if and #endif preprocessor directives will be included for compilation. If the DEBUG symbol has not been defined, the statement — the statement(s) wrapped within the #if and #endif preprocessor — will be omitted from the compilation.

To test out the TestDefine program, follow these steps:

1. Launch the Visual Studio 2008 command prompt (Start→Programs→Microsoft Visual Studio 2008→Visual Studio Tools→Visual Studio 2008 Command Prompt).

2. Change to the path containing the program (C:TestDefine).

3. Compile the application by issuing the command:

csc Program.cs