C#一个小程序出现NullReferenceException

Question

刚刚接触反射技术，但并不理解，对着资料打的程序
namespace 反射
{
class Program
{

    class User
    {
        private int _id;   //模拟一个数据库中User的实体
        public int ID
        {
            get { return _id; }
            set { _id = value; }
        }

        private string _name;
        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }
    }

interface IUser
{
void Insert(User user); //一个从数据库中获取User的接口
User GetUser(int id);

    }

    class SqlserverUser : IUser //继承IUser的sql类
    {
        public void Insert(User user)
        {
            Console.WriteLine("在sql中插入数据");  //模拟的方法
        }

        public User GetUser(int id)
        {

            Console.WriteLine("在sql中得到一条记录");  
            return null;
        }
    }

    class AccessUser : IUser
    {
        public void Insert(User user)
        {
            Console.WriteLine("在Access中插入数据");
        }

        public User GetUser(int id)
        {
            Console.WriteLine("在Access中得到一条记录,{0}",id);
            return null;
        }
    }

    class Department
    {
        private int _id;
        public int ID                   //数据库中的department实体
        {
            get { return _id; }
            set { _id = value; }
        }

        private string _location;
        public string Location
        {
            get { return _location; }
            set { _location = value; }
        }
    }

    interface IDepartment
    {
        void Insert(Department department);  //获取Department的接口

        Department GetDepartment(int id);
    }

    class SqlserverDepartment : IDepartment 
    {
        public void Insert(Department department)   //和上面的差不多
        {
            Console.WriteLine("在sql中插入数据");
        }

        public Department GetDepartment(int id)
        {
            Console.WriteLine("在sql中得到一条记录");
            return null;
        }
    }

    class AccessDepartment : IDepartment
    {
        public void Insert(Department department)
        {
            Console.WriteLine("在Access中插入数据");
        }

        public Department GetDepartment(int id)
        {
            Console.WriteLine("在Access中得到一条记录");
            return null;
        }
    }




    class DataAccess    //这个类完全是照抄资料上的，并不是很理解
    {
        private static readonly string AssemblyName = "反射";
        private static readonly string db = "Sqlserver";

        public static IUser CreateUser()
        {
            string className = AssemblyName + "." + db + "User";
            return (IUser)Assembly.Load(AssemblyName).CreateInstance(className);
        }

        public static IDepartment CreateDepartment()
        {
            string className = AssemblyName + "." + db + "Department";
            return (IDepartment)Assembly.Load(AssemblyName).CreateInstance(className);
        }

    }
    static void Main(string[] args)
    {
        User user = new User();

        IUser iu = DataAccess.CreateUser();

        iu.Insert(user); 

    }
}

}

执行时会抛出NullReferenceException：未将对象引用设置到对象的实例 上面显示的是最后一行代码有问题
​求教各位大神是哪儿出错了 using System.Reflection;在程序里有添加

Answer 1

既然是iu.Insert(user)出了问题，那就应该在Insert的实现方法里面设断点跟踪才是，空指针异常的情况一般发生在 对象实例为空却还调用这个实例的属性或方法 的时候，那就设断点看看iu是不是为空呗，如果不为空就跟踪到Insert方法里面呗，总有一个对象实例是空的却要对他进行操作才会有这个异常的。。。。。。再说下去就太小看你了