如何创建定制异常类？

using System;

public class NoDescException : ApplicationException

{

    public NoDescException() { }

    public NoDescException(string message) : base(message) { }

    public NoDescException(string message, Exception innerEx)

        : base(message, innerEx) { }

}

public interface IFun1

{ string ShowMe();}

public interface IFun2

{ string ShowMe(); }

class Circle : IFun1

{

    public string ShowMe()

    { return "Circle-IFun1"; }

}

public class ObjShowMe

{

    public static void ShowMe(object obj)

    {

        if (!(obj is IFun1 && obj is IFun2))

        {

            throw new NoDescException("Interface not implemented for " + obj.ToString());

        }

    }

}

public class MyApp

{

    static void Main()

    {

        Circle myCir = new Circle();

        try

        {

            ObjShowMe.ShowMe(myCir);

        }

        catch (NoDescException ex)

        {

            Console.WriteLine(ex.Message);

        }

    }

}