#502 – #define and #undef Scope

When you include a #define or #undef preprocessor directive in a file, the scope in which that conditional symbol is defined (or not defined) is limited to that single file.

For example, suppose that we define the symbol QUIET within a file that creates an instance of a Dog and then calls the Bark method of the Dog object.

// Code from Program.cs
#define QUIET

using System;
using DogLibrary;

namespace ConsoleApplication1
    class Program
        static void Main()
            Dog d1 = new Dog("Kirby", 12);

Let’s also suppose that the Dog.Bark method, which exists in a separate file, compiles differently depending on whether the symbol QUIET is defined.

// Code from Dog.cs
        public void Bark()

Because QUIET is defined in Program.cs but not in Dog.cs, the Bark method will use the second (not quiet) line.