未将对象引用设置到对象的实例

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