#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);
            d1.Bark();
        }
    }
}

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()
        {
#if QUIET
            Console.WriteLine("Arf");
#else
            Console.WriteLine("WOOOOOF!");
#endif
        }

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