在敲机房充值记录查询的过程中，当我调用工厂方法，实例化一个DAL层数据库类返回一个充值记录查询的接口，当执行这个接口的方法的过程中出现了如下的错误：未将对象引用设置到对象的实例
出现问题原因：
工厂层：实例化了SqlServerCharge_queryDAL类的一个对象，返回IDAL.IselectDAL接口
'充值记录查询 Public Shared Function Charge_query() As IselectDAL Dim dalUserInfoName As String = AssemblyName & "." & strDB & "Charge_queryDAL" Return CType(Assembly.Load(AssemblyName).CreateInstance(dalUserInfoName), IDAL.IselectDAL) End Function但是在DAL层，我把充值记录查询的方法写在了SelectDAL类里.
'学生充值记录查询 Public Function Charge_query(ByVal cardInfo As JFEntity.CardInfo) As DataTable Implements IselectDAL.Charge_query '传参赋值 sqlParams = {New SqlParameter("@cardno", cardInfo.CardNo)} sql = "select cardno as 卡号,addmoney as 充值金额,date as 充值日期,time as 充值时间,UserID as 充值教师 from ReCharge_Info where cardno= @cardno" table = SQLHelper.ExecSelect(sql, CommandType.Text, sqlParams) Return table End Function 所以导致当我在B层写Icharge_query = JFFactory.Charge_query实例化一个类的对象，但是Charge_query方法却在类SelectDAL里，所以就会出现 没有实例化出来方法所对应的对象错误。
解决办法：
把工厂中把实例化SqlServerCharge_queryDAL的过程改为实例化SelectDAL.如下：
'充值记录查询 Public Shared Function Charge_query() As IselectDAL Dim dalUserInfoName As String = AssemblyName & "." & "SelectDAL" Return CType(Assembly.Load(AssemblyName).CreateInstance(dalUserInfoName), IDAL.IselectDAL) End Function
