反射取值有关问题，求指点

   public class A    {        public int Id { get; set; }        B model = new B();        public B Model        {            get { return model; }            set { model = value; }        }    }    public class B    {        public int Id { get; set; }    }    public void Reflection()    {        A a = new A();        PropertyInfo[] Arr = a.GetType().GetProperties();        Dictionary<string, object> dic = new Dictionary<string, object>();        foreach (PropertyInfo item in Arr)        {            //当获取的属性的类型是一个类时，就会报出“为将对象引用设置到对象的实例”的异常            dic.Add(item.Name,item.GetValue(a,null));        }    }