三层已经学了一些时间了，開始认为自己能够用C#敲代码了，就用C#写了一个实现登陆的，真正再用在机房中，还是认为非常吃力的，所以，决定用vb.net敲了。以下是我用vb.net实现的登陆，能够给大家做一些參考。

  这是构架的三层

  数据库中的表T_User_DAL：UserI，Level，Password，Head，computer

  首先看一下

                                        Entity层：

Public Class UserInfoEntity
#Region "定义变量"
    Private _userID As String
    Private _level As String
    Private _Head As String
    Private _password As String
#End Region
    Public Property UserID() As String            ‘将T_User_DAL表中的每个实体都传上去，实体是依据表来建的，不是功能
        Get
            Return _userID
        End Get
        Set(value As String)
            _userID = value
        End Set
    End Property
    Public Property Level() As String
        Get
            Return _level
        End Get
        Set(ByVal value As String)
            _level = value
        End Set
    End Property
    Public Property Head() As String
        Get
            Return _Head
        End Get
        Set(value As String)
            _Head = value
        End Set
    End Property
    Public Property Password() As String
        Get
            Return _password
        End Get
        Set(value As String)
            _password = value
        End Set
    End Property


End Class

                                        DAL层：

   为了可以减少耦合一定要将连接字符串拿出来，不要每个DAL中都写一边。

Imports System.Data
Imports System.Data.SqlClient
Public Class SqlUtil
    Public Shared Function connstring() As String
        ‘connstring = "Server=192.168.24.183;Database=ReconsitutionCharge_sys;User ID=sa;Password=123456"     ‘会出现无法识别userid的错误
        connstring = "Server=mx; Database=ReconsitutionCharge_sys; User ID=sa; Password=123456"         ‘连接SQL的字符串
    End Function

End Class

T_User_dal表DAL层代码：

Imports System.Data.SqlClient
Imports System.Data
Public Class UserDAL
    Public Function SelectUser(ByVal user As Entity.UserInfoEntity) As Entity.UserInfoEntity
        Dim conn As New SqlConnection                          ‘定义连接打开数据库
        Dim cmd As New SqlCommand                              ‘定义数据库命令
        conn = New SqlConnection(SqlUtil.connstring())         ‘实例化SQLUntil中返回的字符串 
        cmd.Connection = conn


        cmd.CommandText = "Select * From T_User_DAL Where UserID=@UserID and Password=@Password"        ‘存储过程


        cmd.Parameters.Add(New SqlParameter("@UserID", user.UserID))                        ‘赋值，传參
        cmd.Parameters.Add(New SqlParameter("@Password", user.Password))
        cmd.CommandType = CommandType.Text                                                  ‘CommandText 属性设置为存储过程的名称。当调用 Execute 方法之中的一个时，该命令将运行此存储过程
        conn.Open()                                                                         ‘打开连接




        Dim reader As SqlClient.SqlDataReader                                               ‘读取数据库中的表
        reader = cmd.ExecuteReader()


        Dim users As New Entity.UserInfoEntity                                              ‘实例化实体


        While (reader.Read())
            ‘If users Is Nothing Then
            ‘    user = New Entity.UserInfoEntity
            ‘End If
            users.UserID = reader.GetString(reader.GetOrdinal("UserID"))
            users.Password = reader.GetString(reader.GetOrdinal("Password"))
        End While
        conn.Close()
        Return users
    End Function
End Class

                                        BLL层：

Public Class LoginBLL
    Function UserLogin(ByVal user As Entity.UserInfoEntity) As Entity.UserInfoEntity
        Dim uDAL As New DAL.UserDAL                                 ‘实例化了DAL中的UserDAL
        Dim users As New Entity.UserInfoEntity                      ‘实例化了Entity层信息表

        users = uDAL.SelectUser(user)                               ‘selectUser是返回一个Entity类的，给它赋值user
        Return users                                                ‘返回users实体
    End Function

End Class

                                        UI层：

    界面设计：

   Private Sub btnLogin_Click(sender As Object, e As EventArgs) Handles btnLogin.Click
        Dim mgr As New BLL.LoginBLL             ‘实例业务层
        Dim user As New Entity.UserInfoEntity   ‘实例实体
        Dim users As Entity.UserInfoEntity      ‘接收实体，用于接收UserLogin（）返回的数据类型，假设没有New就不是实例化


        user.UserID = txtUserID.Text.Trim()     ‘给实体层传入值
        user.Password = TxtPassword.Text.Trim()
        If txtUserID.Text = "" Then             ‘推断输入是否正确
            MsgBox("username不能为空！")
            Return
        End If

        If TxtPassword.Text = "" Then
            MsgBox("password不能为空！")
            Return
        End If

        Try                                    ‘users接收实体
            users = mgr.UserLogin(user)

            If (users.UserID Is Nothing And users.Password Is Nothing) Then      ‘假设users不为空，则登陆成功！
                MsgBox("登陆失败，username和password不匹配")
                Return
            Else
                MsgBox("登陆成功！登陆用户：" + user.UserID)
            End If
        Catch ex As Exception
            MsgBox(ex.Message())
        End Try
    End Sub
End Class