please define #define 
<p>Could someone give me some examples of how to use #define in c#?</p>

<pre><code>#define //preprocessor directive
</code></pre>

<p>what is the purpose of it? Here example from Microsoft which I still don't get:</p>

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