methods.cs

/Suppose we have few expressions in and parser in library.dll
interface Exp { string Eval(); } 
interface IntExp : Exp {} 
interface AddExp : Exp {}
public interface IParser { Exp Parse(String s); }
public class ParserImpl : IParser { Exp Parse(String s); }
public static class ParserProvider { public static IParser GetParser() => new ParserImpl(); }

//There is client.dll
public static class Client 
{
    public static void Main(string[] args)
    {
        var e = ParserProvider.GetParser().Parse(s[0]);
        Console.WriteLine(e.Eval());
    }
}

//======= Suppose we want to add BoolExp with Multimethods =======
//(1) Add bool exp
interface BoolExp : Exp {}
//(2) Modify parser to support bool. ParserProvider returns modified parser.
public class ParserImpl : IParser { Exp Parse(String s); }
//DONE, client code remains same, just change library version.