Programming C#, Third Edition

Preprocessor Directives

In the examples you’ve seen so far, you’ve compiled your entire program whenever you compiled any of it. At times, however, you might want to compile only parts of your program—for example, depending on whether you are debugging or building your production code.

Before your code is compiled, another program called the preprocessor runs and prepares your program for the compiler. The preprocessor examines your code for special preprocessor directives, all of which begin with the pound sign (#). These directives allow you to define identifiers and then test for their existence.

Defining Identifiers

#define DEBUG defines a preprocessor identifier, DEBUG. Although other preprocessor directives can come anywhere in your code, identifiers must be defined before any other code, including using statements.

Tip

C and C++ programmer take note: The C# preprocessor implements only a subset of the C++ preprocessor.

You can test whether DEBUG has been defined with the #if statement. Thus, you can write:

#define DEBUG

//... some normal code - not affected by preprocessor

#if DEBUG
   // code to include if debugging
#else
  // code to include if not debugging
#endif

//... some normal code - not affected by preprocessor

When the preprocessor runs, it sees the #define statement and records the identifier DEBUG. The preprocessor skips over your normal C# code and then finds the #if - #else - #endif block.

The #if statement tests for the identifier DEBUG, which does exist, and so ...