机房收费系统中，对于其需求而言，其中少不了要用到datagridview控件来显示一些从数据库中检索到的信息。现在用其中的一个实例来实现以下系统中查询用户充值记录的用例。

　　以前自己学习的三层资料中，得到的一个学习信息是在三层中，应该把业务逻辑层作为出发点，然后进行功能的实现。或许一个人一个感觉吧，我总是感觉从数据访问层出发，更符合我们的思维逻辑，从而方便业务功能的实现。对于用户充值记录。

Dal数据访问层：

ImportsSystem.Data.sqlClient

Public Function Checkrecord(ByVal Student AsEntity.RechargeRecord) As DataTable

Dim conn As sqlConnection = NewsqlConnection(DAL.DbConnection.Constr)

Dim da As New sqlDataAdapter

Dim dst As New DataSet

Dim dt As New DataTable

conn.Open()

Dim sql As String = "select *from Recharge_Info where CardID='"& Student.CardID & "'"

Dim cmd As sqlCommand = NewsqlCommand(sql,conn)

da.SelectCommand = cmd

da.Fill(dst,"Recharge_Info")

dt =dst.Tables("Recharge_Info")

conn.Close()

Return dt

End Function

BLL业务逻辑层:

PublicFunction CheckRecord(ByVal Student As Entity.RechargeRecord) As DataTable

Dim dalRechargeRecord As NewDAL.RechargeRecordDAL

Dim dt As New DataTable

dt =dalRechargeRecord.Checkrecord(Student)

Return dt

End Function

UI用户界面层：

Dim entityStuRecordAs New Entity.RechargeRecord

entityStuRecord.CardID = txtCardID.Text

Dim bllRechargeRecord As NewBLL.RechargeRecord

DataGridView1.DataSource =bllRechargeRecord.CheckRecord(entityStuRecord)