#undef (C# Reference)

#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.

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;  
class MyClass
{  
    static void Main()
    {  
#if DEBUG  
        Console.WriteLine("DEBUG is defined");  
#else  
        Console.WriteLine("DEBUG is not defined");  
#endif  
    }  
}  

DEBUG is not defined

See Also