#undef lets you undefine a symbol, such that, by using the symbol as the expression in a #if directive, the expression will evaluate to false.

#undef symbol

where:

symbol

The name of the symbol you want to undefine.

Remarks

A symbol can be defined either with the #define directive or the /define compiler option. The #undef directive must appear in the file before you use any statements that are not also directives.

Example

// preprocessor_undef.cs
// compile with: /d:DEBUG
#undef DEBUG
using System;
public class MyClass 
{
   public static void Main() 
   {
      #if DEBUG
         Console.WriteLine("DEBUG is defined");
      #else
         Console.WriteLine("DEBUG is not defined");
      #endif
   }
}

Output

DEBUG is not defined

See Also

C# Preprocessor Directives