Question

What are some examples of how to use #define in C#?

#define //preprocessor directive

What is the purpose of it? Here is an example from Microsoft which I still don’t get:

// preprocessor_if.cs
#define DEBUG
#define MYTEST
using System;
public class MyClass
{
    static void Main()
    {
#if (DEBUG && !MYTEST)
        Console.WriteLine("DEBUG is defined");
#elif (!DEBUG && MYTEST)
        Console.WriteLine("MYTEST is defined");
#elif (DEBUG && MYTEST)
        Console.WriteLine("DEBUG and MYTEST are defined");
#else
        Console.WriteLine("DEBUG and MYTEST are not defined");
#endif
    }
}

Answer 1

Quote:

The #define and #undef lines should appear at the very top of a source
text file and they can adjust compilation options for the entire file.

and

In the C# language, the #define line is considered a preprocessing
directive. There are some invalid syntaxes for defined symbols; you
cannot use a number value as the defined identifier, for example.

and

The #undef directive ensures that after the textual point in the file,
the specified identifier is not defined.

In your example, if DEBUG is defined (does not matter what it is as long as it is defined) and MYTEST is not defined, then it will show the message shown.

Otherwise, if DEBUG is not defined but MYTEST is defined, then it will show the message shown.

If both are defined then it will show the message shown.

Bottom-line:

The point of the defines is to selectively apply compilation options to your program to give it a different program flow. It is a carryover from C and C++.