How do I make a sealed class?

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I am trying to make a sealed class from following codes. But not understanding how to add functionality to by extending the the class using a static class with static members (functions, properties, or indexers). I am not getting any errors but, I don't think it is a proper sealed class.

{
   sealed class Magic
    {
        private int toads;
        private int spiders;

        public Magic()
        {
            toads = 0;
            spiders = 0;
        }

        public int Toads
        {
            get { return toads; }
            set { toads = value; }
        }
        public int Spiders
        {
            get { return spiders; }
            set { spiders = value; }
        }
        public Magic(int t, int s)
        {
            Toads = t;
            Spiders = s;
        }
        public void doMagic()
        {
            Console.WriteLine("Poof!");

        }
      }
    static class ExtendMagic 
    {
      
    }
    class Program
    {
        static void Main()
        {
            Magic yourMagic = new Magic();
            Console.WriteLine("Spiders = {0} and Toads = {1}", yourMagic.Spiders, yourMagic.Toads);
            Magic mymagic = new Magic(3, 5);
            mymagic.doMagic();
            Console.WriteLine("Spiders = {0} and Toads = {1}", mymagic.Spiders, mymagic.Toads);
            Console.ReadKey();

        }
    }
}

Posted 3-May-14 19:56pm

RalvarezHose

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2 solutions

Solution 1

The question makes no sense and looks more than pointless. This is a properly sealed class. You did not demonstrate extending this class, did not even try; and you cannot do so.

—SA

Posted 3-May-14 20:06pm

Sergey Alexandrovich Kryukov

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OriginalGriff · Accepted Answer · 2014-05-03T20:12:00

When you use the sealed keyword on a class, it prevents any class being derived from it; you can't do this:

C#

public sealed class A {}
public class B : A {}

It doesn't prevent extension methods being declared because they do not derive from the class, but extend it.
Extension methods do not get any additional access to the class internals, they just allow a syntactic sugar so you can add methods that look like part of the original class. For example, the DateTime class does not have a method to return the first of the month, so you can do this:

C#

DateTime firstOfMonth;
DateTime now = DateTime.Now;
firstOfMonth = (now.AddDays(1 - now.Day)).AtMidnght();

Or include one:

C#

/// <summary>
/// Returns the first day of the month
/// </summary>
/// <example>
/// DateTime firstOfThisMonth = DateTime.Now.FirstOfMonth;
/// </example>
/// <param name="dt">Start date</param>
/// <returns></returns>
public static DateTime FirstOfMonth(this DateTime dt)
    {
    return (dt.AddDays(1 - dt.Day)).AtMidnight();
    }

Now your code can use the clearer format:

C#

DateTime firstOfMonth = DateTime.Now.FirstOfMonth();