Preprocessor Directives

The last topic discussed in this chapter is using preprocessor directives . These are directives to the compiler, not C# statements, and they enable you to skip compilation of sections of your code that you specify.

Here are the C# preprocessing directives:

• #define and #undef are used to define and undefine, respectively, conditional compilation symbols.

• #if , #elif , #else , and #endif are used to conditionally skip sections of source code.

• #error and #warning are used to display errors and warnings, respectively.

• #region and #endregion are used to mark sections of source code.

• #line is used to control line numbers for errors and warnings.

You can see an example in ch01_12.cs, Listing 1.12. In that example, it's the user 's birthday, so we'll display a birthday greeting. To do that, we define a symbol named Birthday and expressly undefine a symbol named NormalDay . Then we can use #if statements to indicate which code we want compiled and which we want to omit, and in this case, we'll only compile a statement to display the birthday greeting.

FOR C++ PROGRAMMERS

In C++, preprocessor directives are evaluated in a preliminary run-through of your code first, and then the code itself is compiled. In C#, on the other hand, all this takes place in the same pass through your code (in fact, it's not "preprocessing" at all).

Listing 1.12 Using Preprocessor Directives (ch01_12.cs)

 #define Birthday #undef NormalDay using System; class ch01_12 {   static void Main()   {     #if Birthday       Console.WriteLine("Happy Birthday!");     #else       Console.WriteLine("It's not your birthday.");     #endif     #if NormalDay       Console.WriteLine("It's just a normal day.");     #endif   } } 

Because we've defined the symbol Birthday , this code in Listing 1.12 makes the compiler compile the statement Console.WriteLine("Happy Birthday!"); , but not the statement Console.WriteLine("It's not your birthday."); :

 #if Birthday   Console.WriteLine("Happy Birthday!"); #else   Console.WriteLine("It's not your birthday."); #endif 

When you compile ch01_12.cs, the only WriteLine statement that is actually compiled is Console.WriteLine("Happy Birthday!"); , so here's the result when you run this program:

 C:\>ch01_12 sHappy Birthday!