//ICar.dll
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ICar
{
    public interface ICar
    {
        string GetCar();
    }
}


//FuTe.dll
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace FuTe
{
    public class FuTe : ICar.ICar
    {
   
        public string GetCar()
        {
            return "FuTe";
        }
    }
}


//CarFactory.dll
using System;
using System.Reflection;

namespace CarFactory
{
    public class CarFactory
    {

        public static ICar.ICar CreateCar(string CarFileName)
        {
            var CarAssembly = Assembly.LoadFrom(CarFileName);
            ICar.ICar CarType = null;
            foreach (var tmpCarType in CarAssembly.GetTypes())
            {
                    CarType = (ICar.ICar)Activator.CreateInstance(tmpCarType);
                    break;
            }
            return CarType;
        }
    }
}

//ConsoleApplication6.exe
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication6
{
    class Program
    {
        static void Main(string[] args)
        {
            var myCar = CarFactory.CarFactory.CreateCar(@"FuTe.dll");
            var CarName = myCar.GetCar();
            Console.WriteLine(CarName);
            Console.ReadLine();
        }
    }
}

如果CarFactory.CarFactory.CreateCar(这里是相对路径,那么请把ConsoleApplication6.exe和所有dll放在同一个文件夹)
此例子简单,我觉得足以表达思想.
有不明白请提问.