三层例子

                    敲一个例子比登天还难，一头雾水，无从下手啊。我的理解能力。。。。那就从最简单地三层登陆例子开始吧。

     传递的是实体，userinfo，而不是具体的参数。

     首先，UI层需要添加引用Entity和BLL层，BLL层需要添加引用DAL和Entity，而DAL需要添加引用Entity。Entity中定义了很多的参数，这些参数在三层之间传递。

      按照引用的顺序，我们先需要敲entity层，他里面的参数是数据库中的字段，按着敲就可以啦。接着需要敲DAL层，他里面定义了数据库的连接。之后就是BLL层啦，接着就是UI层。

首先，解决方案里面的设计：

              

接着，来看Entity层：

     

Public Class Userinfo
    Private _UserID As String
    Private _level As String
    Private _PWD As String
    Private _Statues As String
    Private _Username As String
    Public Property UserID As String
        Get
            Return _UserID
        End Get
        Set(value As String)
            _UserID = value
        End Set
    End Property
    Public Property PWD As String
        Get
            Return _PWD
        End Get
        Set(value As String)
            _PWD = value
        End Set
    End Property
    Public Property Statues As String
        Get
            Return _Statues
        End Get
        Set(value As String)
            _Statues = value
        End Set
    End Property
    Public Property Level As String
        Get
            Return _level
        End Get
        Set(value As String)
            _level = value
        End Set
    End Property
    Public Property Username As String
        Get
            Return _Username
        End Get
        Set(value As String)
            _Username = value
        End Set
    End Property
End Class

      里面和数据库中的字段都是一一对应的。可读可写。

接下来就是DAL层：

Imports System.Data
Imports System.Data.SqlClient
Imports Entity

Public Class UserLogin
    Public conn As New SqlConnection("server=.;database=computer;user ID=sa;PWD=123456")
    Public Function SelectUser(ByVal User As Entity.Userinfo) As Entity.Userinfo
        '传递实体userinfo，而不是参数ID，username等，这样方便对实体等参数进行调用
        Dim reader As SqlDataReader  '定义类型为sqldatareader的变量reader
        Dim eUser As New Entity.Userinfo      '实例化新的userinfo
        Dim sql As String = "select UserID,PWD from User_info where UserID=@username and PWD=@password "
        Dim cmd As New SqlCommand(sql, conn)    '创建sqlcommand对象

        cmd.CommandText = sql   '获取sql语句的具体内容
        cmd.CommandType = CommandType.Text
        cmd.Parameters.Add(New SqlParameter("@username", User.UserID))
        cmd.Parameters.Add(New SqlParameter("@password", User.PWD))
        conn.Open() '打开数据库

        reader = cmd.ExecuteReader  '执行查询语句，并生成DataReader
        While reader.Read()   '获取数据库中相应字段的数据
            eUser.UserID = reader.GetString(0)
            eUser.PWD = reader.GetString(1)
        End While
        Return eUser  '返回查询到的实体
        conn.Close()    '关闭连接
    End Function
End Class

我们再来看业务逻辑层BLL:

Public Class LoginManager
    Public Function UserLogin(ByVal User As Entity.Userinfo) As Entity.Userinfo
        Dim uDao As New DAL.UserLogin
        Dim eUser1 As New Entity.Userinfo
        eUser1 = uDao.SelectUser(User) '将数据传给D层
        If IsNothing(eUser1.UserID) Then
            Throw New Exception("登陆失败，请验证登录名和密码")
        Else
            MsgBox("登陆成功")
        End If
    End Function
End Class

UI层就是展示给我们看的，需要把它设置成为启动项：

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Try
            Dim euser2 As New Entity.Userinfo
            Dim euser3 As New Entity.Userinfo

            euser2.UserID = TextBox1.Text.Trim '将U层数据传递给实体
            euser2.PWD = TextBox2.Text.Trim

            Dim mgr As New BLL.LoginManager '将数据传递给B层
            euser3 = mgr.UserLogin(euser2)
        Catch ex As Exception
            MessageBox.Show(ex.Message.ToString())
        End Try
    End Sub
End Class

就这样，登陆成功啦。

                         

      对于三层的理解，还需要多多的实践啊。。。。。。