using System;    

​

public enum PageType

{

    Closing,

    Error,

    AppraiserRun

};

public class Program

{

    public static void Main()

    {

        Console.WriteLine(Format(PageType.Closing));

        Console.WriteLine(Format(PageType.Error));

        Console.WriteLine(Format(PageType.AppraiserRun));

        Console.WriteLine(Format());

    }

    public static string Format(PageType? pageType = null)

    {

        switch (pageType) {

            case PageType.Error:

                return "Error";

            case PageType.Closing:

                return "Closing";

            case null:

                return "NULL";

            default:

                return "Unknown";

        }

    }

}