C# 2008 Programmer's Reference

4. Run the program by issuing the command:

Program.exe

Figure 3-13 shows the output of the application. As you can see, the debugging statement prints out the intermediate results.

Figure 3-13

To undefine a symbol, you can use the #undef preprocessor directive, like this:

#undef DEBUG
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
...

If you recompile the program now, the debugging statement will be omitted.

Another popular way of using the #define preprocessor directive is to omit the definition of the symbol and inject it during compilation time. For example, if you remove the #define preprocessor directive from the program, you can define it using the /define compiler option:

1. In Visual Studio 2008 command prompt, compile the program using:

csc Program.cs /define:DEBUG

2. Run the program by issuing the command:

Program.exe

The output is identical to what you saw in Figure 3-13 — the debugging statement prints out the intermediate results.

If you now recompile the program by defining another symbol (other than DEBUG), you will realize that the debugging output does not appear (see Figure 3-14).

Figure 3-14 

#if, #else, #elif, and #endif

As you saw in the preceding section, the #if and #endif preprocessor directives defines a block of code to include for compilation if a specified symbol is defined. You can also use the #else and #elif preprocessor directives to create compound conditional directives.

Using the previous example, you can add the #else and #elif preprocessor directives as follows:

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);
#elif NORMAL
     Console.WriteLine('sum={0}', sum);
#else
     Console.WriteLine('.');
#endif
    }
   }
   Console.WriteLine(
    'Sum of all odd numbers from 1 to {0} is {1}',
     num, sum);
   Console.ReadLine();
  }
 }
}

Figure 3-15 shows the different output when different symbols are defined. The top screen shows the output when the DEBUG symbol is defined. The middle screen shows the output when the NORMAL symbol is defined. The bottom screen shows the output when no symbol is defined.

Figure 3-15

The #if preprocessor directive can also test for multiple conditions using the logical operators. Here are some examples:

#if (DEBUG || NORMAL) //---either DEBUG or NORMAL is defined---
#if (DEBUG && NORMAL) //---both DEBUG and NORMAL are defined---
#if (!DEBUG && NORMAL) //---DEBUG is not defined AND NORMAL is defined---

#warning and #error

The #warning preprocessor directive lets you generate a warning from a specific location of your code. The following example shows how you can use it to display warning messages during compilation time.

for (int i = 1; i <= num; i++) {
 //---sum up all odd numbers---
 if (i % 2 == 1) {
  sum += i;
#if DEBUG
#warning Debugging mode is on
  Console.WriteLine('i={0}, sum={1}', i, sum);
#elif NORMAL
#warning Normal mode is on
  Console.WriteLine('sum={0}', sum);
#else
#warning Default mode is on