#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