C#中实现登录功能的完整步骤

-- 使用master 数据库
use master
go
if exists(select *from sysdatabases where name="StudentDB")
drop database StudentDB
go
create database StudentDB
go

use StudentDB
go

-- 新建学生表
if exists (select *from sysobjects where name="Student")
drop table Student
go
create table Student
(
stuId   int primary key identity(2000,1),
stuName varchar(30) not null,
stuSex  char(2) not null,
stuAge  int not null,
stuTel  varchar(11) not null,
stuPWd  varchar(30) not null
)

-- 添加学生表的数据
insert into Student (stuName,stuSex,stuAge,stuTel,stuPWd)values("张三","男",21,"12345678543","123456")
insert into Student (stuName,stuSex,stuAge,stuTel,stuPWd)values("赵六","男",21,"12345678543","123456")
insert into Student (stuName,stuSex,stuAge,stuTel,stuPWd)values("韩菲","女",20,"12345678543","123456")


 -- 新建教师表
if exists (select *from sysobjects where name="Teacher")
drop table Teacher
go
create table Teacher
(
tId   int primary key identity(4000,1),
tName varchar(30) not null,
tSex  char(2) not null,
tAge  int not null,
tTel  varchar(11) not null,
tTitle varchar(20),
tPwd   varchar(30) not null    
)
--添加教师表
insert into Teacher(tName,tSex,tAge,tTel,tTitle,tPwd)values("xx","男",32,"12345678901","副教授","123456")

 -- 新建管理员
if exists (select *from sysobjects where name="Admin")
drop table Admin
go
create table Admin
(
adminId   int primary key identity(4000,1),
adminName varchar(30) not null,
adminPWd  varchar(30) not null
)
-- 添加管理员表
insert into Admin(adminName,adminPwd) values("admin","123456")

 public  class Admin
  {
        public int adminId { get; set; }
        public String adminName { get; set; }
        public String adminPwd { get; set; }
  }

public class Student
    {
        public int stuId { get; set; }
        public string stuName { get; set; }
        public string stuSex { get; set; }
        public int stuAge { get; set; }
        public string stuTel { get; set; }
        public string stuPwd { get; set; }


    }

 public class Teacher
 {
        public int tId { get; set; }
        public string tName { get; set; }
        public string tSex { get; set; }
        public int tAge { get; set; }
        public string tTel { get; set; }
        public string tTitle { get; set; }
        public string tPWd { get; set; }
 }

public class DbHelper
    {
        /// 

        /// 获取连接对象
        /// 
        /// 
        public static SqlConnection GetConnection()
        {

            SqlConnection cOnn= null;
            try
            {
                //可能发生错误的代码
                if (cOnn== null)
                {
                    cOnn= new SqlConnection();
                    conn.COnnectionString= ConfigurationManager.ConnectionStrings["connString"].ToString();
                    conn.Open();
                    conn.Close();
                }
                return conn;
            }
            catch (Exception ex)
            {
                //发生异常以后要做的事情
                throw ex; // 把问题抛出，让程序员知道那里出了错误
            }
        }

        /// 

        /// 执行增删改
        /// 
        /// 
        /// 
        public static int GetExcuet(string sql)
        {
            // 1. 获取连接对象
            SqlConnection cOnn= GetConnection();
            try
            {
                // 2.打开链接
                conn.Open();
                //3.创建SqlCommand对象,sql语句，连接对象
                SqlCommand cmd = new SqlCommand(sql, conn);
                // 4.执行SQL,并返回受影响的行数
                return cmd.ExecuteNonQuery(); ;
            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {
                conn.Close();
                conn.Dispose();
            }
        }

        /// 

        /// 返回执行查询的结果
        /// 
        /// 
        public static DataTable GetDataSet(String sql)
        {
            try
            {
                // 1.获取链接对象
                SqlConnection cOnn= GetConnection();
                //2.创建适配器对象
                SqlDataAdapter da = new SqlDataAdapter(sql, conn);
                //3.创建DataSet 对象
                DataSet ds = new DataSet();
                da.Fill(ds);
                return ds.Tables[0];
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
       
      
    }

           //获取用户名和密码
            string username = txtUserName.Text.Trim();
            string pwd = txtPwd.Text.Trim();
            //当用户名为空的时候就不往下面执行了
            if (username.Equals(""))
            {
                MessageBox.Show("用户名不能为空");
                return;
            }
            if (pwd.Equals(""))
            {
                MessageBox.Show("密码不能为空");
                return;
            }

           //管理员登录
            if (radAdmin.Checked)
            {
                AdminLogin(username);
            }
            //学生登录
            if (radStudent.Checked)
            {
               StudentLogin();
            }
            //教师登录
            if (radTeacher.Checked)
            {
                TeacherLogin();
            }

  private Admin AdminLogin(String username)
        {

            string sql = string.Format("select *from Admin where adminId={0}",username);
            DataTable table=  DbHelper.GetDataSet(sql);
            //判断表的行数,大于等于1表示有数据，用户存在,否则返回null
            if (table.Rows.Count <1) return null;

            //新建一个admin对象
            Admin admin = new Admin();
            admin.adminId = Convert.ToInt32(table.Rows[0]["adminId"]);
            admin.adminName = table.Rows[0]["adminName"].ToString();
            admin.adminPwd = table.Rows[0]["adminPwd"].ToString();
            return admin;
        }

 private Student StudentLogin(string username)
        {
            string sql = string.Format("select *from Student where stuId={0}", username);
            DataTable table = DbHelper.GetDataSet(sql);
            //判断表的行数,大于等于1表示有数据，用户存在,否则返回null
            if (table.Rows.Count <1) return null;

            /*新建一个student对象 ，这里只给了三个字段进行了赋值，
             * 因为我们登录的时候，只用到了id和密码，
             * 其他时候根据需求进行赋值
            */
            Student student = new Student();
            student.stuId = Convert.ToInt32(table.Rows[0]["stuId"]);
            student.stuName = table.Rows[0]["stuName"].ToString();
            student.stuPwd = table.Rows[0]["stuPwd"].ToString();
            return student;
        }

  private Teacher TeacherLogin(string username)
        {
            string sql = string.Format("select *from Teacher where tId={0}", username);
            DataTable table = DbHelper.GetDataSet(sql);
            //判断表的行数,大于等于1表示有数据，用户存在,否则返回null
            if (table.Rows.Count <1) return null;

            /*新建一个student对象 ，这里只给了三个字段进行了赋值，
             * 因为我们登录的时候，只用到了id和密码，
             * 其他时候根据需求进行赋值
            */
            Teacher teacher = new Teacher();
            teacher.tId = Convert.ToInt32(table.Rows[0]["tId"]);
            teacher.tName = table.Rows[0]["tName"].ToString();
            teacher.tPWd = table.Rows[0]["tPWd"].ToString();
            return teacher;
        }

 private void btnLogin_Click(object sender, EventArgs e)
   {
       //获取用户名和密码
       string username = txtUserName.Text.Trim();
       string pwd = txtPwd.Text.Trim();
       if (username.Equals(""))
       {
           MessageBox.Show("用户名不能为空");
           return;
       }
       if (pwd.Equals(""))
       {
           MessageBox.Show("密码不能为空");
           return;
       }
       //管理员登录
       if (radAdmin.Checked)
       {
           /*为什么要返回来，因为以后处理逻辑可能在不同类里面，
            * 这里只是模拟进行分层操作*/
          Admin admin=  AdminLogin(username);
          if (admin == null)
          {
               MessageBox.Show("用户不存在");
               return;
           }
          if (!admin.adminPwd.Equals(pwd))
          {
               MessageBox.Show("密码错误");
               return;
          }
     
       }
       //学生登录
       if (radStudent.Checked)
       {
           Student  student= StudentLogin(username);
           if (student == null)
           {
               MessageBox.Show("用户不存在");
               return;
           }
           if (!student.stuPwd.Equals(pwd))
           {
               MessageBox.Show("密码错误");
               return;
           }
       }
       //教师登录
       if (radTeacher.Checked)
       {
           Teacher teacher=  TeacherLogin(username);

           if (teacher == null)
           {
               MessageBox.Show("用户不存在");
               return;
           }
           if (!teacher.tPWd.Equals(pwd))
           {
               MessageBox.Show("密码错误");
               return;
           }
       }

       MessageBox.Show("登录成功");
   }