C# FAQ: What is the define preprocessor directive

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What is the #define preprocessor directive?

As a member of the C family of programming languages, C# supports preprocessor directives—e.g., #define, #if, and #endif. (Technically speaking, csc.exe does not have a preprocessor; because, these symbols are resolved during the compiler's lexical analysis phase.)

The #define directive enables the creation of custom symbols for controlling code compilation. Unlike C++, the #define directive of C# does not allow you to create macro-style code. Once a symbol has been defined, the #if and #endif directives may be employed to test for that symbol.

The following code represents a typical usage for #define:

#define DEBUG
 
using System;
 
public class TestClass
{
   public static void Main()
   {
      // conditional compilation based on the DEBUG symbol
      #if DEBUG
        Console.WriteLine ("The DEBUG symbol has been defined!");
      #endif
   }
}

When using the #define directive, the symbol is only available within the source file in which it has been defined. However, to define project-wide symbols, simply access the project's property page, navigate to "Configuration Properties | Build", and edit the "Conditional Compilation Constants" field.

In order to disable a constant for a specific file, use the #undef directive.

Also, the C# preprocessor directives are available from ASP.NET as well.

See also

• Does C# have macros or a preprocessor?
• Is there C# support for C-type macros?
• What is the define preprocessor directive?