//第一步创建一个AbstractFactory类，在这个类里面定义两个静态属性：AssemblyPath ，NameSpace 
//第二步在工厂类里面写一个静态方法：public static IUserInfoDal CreateUserInfoDal(){}
//第三步再写一个方法：private static object CreateInstance(string fullName)通过Assembly反射来创建实例
//第四步在web.config文件下配置这两个静态属性：AssemblyPath ，NameSpace

public class AbstractFactory
    {
        private static readonly string AssemblyPath = ConfigurationManager.AppSettings["AssemblyPath"];
        private static readonly string NameSpace = ConfigurationManager.AppSettings["NameSpace"];
        public static IUserInfoDal CreateUserInfoDal()
        {
            string fullName = NameSpace + ".UserInfoDal";
            return CreateInstance(fullName) as IUserInfoDal;
        }

        private static object CreateInstance(string fullName)
        {
            Assembly assembly = Assembly.Load(AssemblyPath);
            return assembly.CreateInstance(fullName);
        }
    }