How to use #undef directive in C#?

The #undef directive in C# allows you to undefine a previously defined symbol, making it unavailable for use in conditional compilation directives like #if. This is useful for controlling which code sections are included during compilation.

Syntax

Following is the syntax for using the #undef directive −

#undef SYMBOL

Where SYMBOL is the identifier you want to undefine. For example −

#undef DEBUG
#undef TESTING

Key Rules

• The #undef directive must appear at the top of the file, before any code or using statements.

• You can only undefine symbols that were previously defined with #define in the same file.

• An undefined symbol evaluates to false in conditional compilation directives.

• Symbols defined via compiler options (like /define) cannot be undefined with #undef.

Using #undef with Conditional Compilation

The #undef directive works together with #if, #elif, and #else directives to control conditional compilation −

#define DEBUG
#define TESTING
#undef TESTING

using System;

class Program {
   static void Main() {
      #if DEBUG
      Console.WriteLine("Debug mode is enabled");
      #endif

      #if TESTING
      Console.WriteLine("Testing mode is enabled");
      #else
      Console.WriteLine("Testing mode is disabled");
      #endif

      #if (DEBUG && TESTING)
      Console.WriteLine("Both DEBUG and TESTING are defined");
      #elif (DEBUG && !TESTING)
      Console.WriteLine("DEBUG is defined, TESTING is undefined");
      #else
      Console.WriteLine("Neither DEBUG nor TESTING are defined");
      #endif
   }
}

The output of the above code is −

Debug mode is enabled
Testing mode is disabled
DEBUG is defined, TESTING is undefined

Practical Example with Feature Flags

Here's a practical example showing how #undef can be used to disable specific features −

#define FEATURE_LOGGING
#define FEATURE_CACHING
#undef FEATURE_CACHING

using System;

class FeatureManager {
   static void Main() {
      Console.WriteLine("Application started");

      #if FEATURE_LOGGING
      LogMessage("Feature logging is active");
      #endif

      #if FEATURE_CACHING
      EnableCaching();
      #else
      Console.WriteLine("Caching is disabled");
      #endif

      Console.WriteLine("Application ended");
   }

   #if FEATURE_LOGGING
   static void LogMessage(string message) {
      Console.WriteLine($"LOG: {message}");
   }
   #endif

   #if FEATURE_CACHING
   static void EnableCaching() {
      Console.WriteLine("Caching system initialized");
   }
   #endif
}

The output of the above code is −

Application started
LOG: Feature logging is active
Caching is disabled
Application ended

Common Use Cases

• Disabling debug features in release builds

• Temporarily removing features without deleting code

• Creating different build configurations for testing and production

• Platform-specific code compilation based on target environment

Conclusion

The #undef directive provides fine-grained control over conditional compilation by allowing you to undefine symbols. It's particularly useful for managing feature flags and creating different build configurations without maintaining separate code files.