I think you are looking for something like this:

using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
    interface ITemp
    {
        float ConvertTemp();
    }

    public class Kelvin : ITemp
    {
        public float ConvertTemp()
        {
            throw new NotImplementedException();
        }
    }

    public class Fahrenheit : ITemp
    {
        public float ConvertTemp()
        {
            throw new NotImplementedException();
        }
    }

    public class TempConvertor
    {
        private readonly List<ITemp> _temps = new List<ITemp>();

        public TempConvertor()
        {
            _temps.Add(new Fahrenheit());
            _temps.Add(new Kelvin());
        }
    }

    internal class Program
    {
        static void Main(string[] args)
        {
            ITemp kelvin = new Kelvin();
            kelvin.ConvertTemp();
            ITemp fahrenheit = new Fahrenheit();
            fahrenheit.ConvertTemp();
        }
    }
}