using System;
using System.Configuration;
using SingerLibrary;

namespace Factory
{

    public class SingerFactory
    {
        public static Singer GetSinger() 
        {
           return null; //for now
        }
    }

}

namespace SingerLibrary
{
    
    public interface Singer
    {
        void Sing();
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
      <add key="GreatSinger" value="SingerLibrary.Elvis" />
      <add key="DuffSinger" value="SingerLibrary.James" />
  </appSettings>
</configuration>

    public class SingerFactory
    {
        public static Singer GetSinger() 
        {
            // Normally you would have a better mechanism
            // than a random number generator, but it is only
            // an example
            if (new Random().Next(10)< 5) 
            {
                return GetSinger("GreatSinger");
            }
            else 
            {
                return GetSinger("DuffSinger");
            }

        }

        private static Singer GetSinger(string type) 
        {
            // No error handling in this method :-))
            // It's just an example, normally I say 
            // 'removed for clarity', hopefully you fall for it.

            string concreteType =  ConfigurationSettings.AppSettings.Get(type);
            string assembly = "SingerLibrary";
            return (Singer) Activator.CreateInstance(assembly,concreteType).Unwrap();
        }
   }