using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public interface IHaveId {

        int id { get; }

    }

    public class Thing1 : IHaveId {

        private int _id;

        public Thing1(int id) {

            this._id = id;

        }

        int IHaveId.id {

            get { return this._id; }

        }

    }

    public class Thing2 : IHaveId {

        private int _id;

        public Thing2(int id) {

            this._id = id;

        }

        int IHaveId.id {

            get { return this._id; }

        }

    }   

    public static int getMaxId<T>(List<T> list) where T : IHaveId {

        return list.Max(i => i.id);

    }

    public static void Main()

    {

        List<IHaveId> things = new List<IHaveId>();

        for (var i=0; i<5; i++) {

            things.Add(new Thing1(i));

        }

        for (var i=10; i<15; i++) {

            things.Add(new Thing2(i));

        }

        Console.WriteLine("Max id is " + getMaxId(things));

    }

}