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   源码介绍

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首先附加数据库，

管理员账号 admin 密码 00000

教师账号 t001 密码 12345

学生账号 SunPeng 密码 12345

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
//下载自：好例子网 http://www.haolizi.net
namespace LoginFrame
{
    public partial class LoginForm : Form
    {
        public LoginForm()
        {
            InitializeComponent();
        }

        private void pictureBox2_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void pictureBox1_Click(object sender, EventArgs e)
        {



            if (ValidateInput())
            { 
                string userName = txtUserName.Text;
                string pwd = txtPwd.Text;
               

                try
                {
                    string sql = "";
                    if (cbxUserStyle.Text == "教员")
                    { 
                         sql =string.Format( "select count(*) from Teacher where LoginId='{0}' and LoginPwd='{1}'",userName,pwd);
                    }
                    else if (cbxUserStyle.Text == "管理员")
                    {
                         sql = string.Format("select count(*) from Admin where  LoginId='{0}' and LoginPwd='{1}'",userName,pwd);
                    }
                    else {//学生
                         sql = string.Format("select count(*) from Student where  LoginId='{0}' and LoginPwd='{1}'", userName, pwd);
                    }
                    

                    SqlCommand command = new SqlCommand(sql,DbHelper.connection);
                    DbHelper.connection.Open();

                    int i = (int)command.ExecuteScalar();
                    if (i > 0)
                    {
                        if (cbxUserStyle.Text == "教员")
                        {
                            SirForm sf = new SirForm();
                            sf.FormClosed  = Sf_FormClosed;
                            sf.Show();
                            this.Hide();
                        }
                        else if (cbxUserStyle.Text == "管理员")
                        {
                            AdminForm af = new AdminForm();
                            af.FormClosed  = Sf_FormClosed;
                            af.Show();
                            this.Hide();
                        }
                        else {
                            StuForm sf = new StuForm();
                            sf.FormClosed  = Sf_FormClosed;
                            sf.Show();
                            this.Hide();
                        }
                        MessageBox.Show("登陆成功！");
                    }
                    else {
                        MessageBox.Show("用户名或密码不正确！");
                        txtPwd.Text = "";
                    }
                    

                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
                finally
                {
                    DbHelper.connection.Close();

                }
                

            }
           
             
        }

        private void Sf_FormClosed(object sender, FormClosedEventArgs e)
        {
            Application.Exit();
        }

        private bool ValidateInput()
        {
            if (txtUserName.Text.Trim() == "")
            {
                MessageBox.Show("请输入用户名！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);

                return false;
            }
            else if (txtPwd.Text.Trim() == "")
            {
                MessageBox.Show("请输入密码！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                return false;
            }
            else if (cbxUserStyle.Text.Trim() == "")
            {
                MessageBox.Show("请选择用户类型！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                return false;
            }
            return true;
        }

        private void LoginForm_Load(object sender, EventArgs e)
        {
            //string sql = "select * from UserType";
            //DataSet dataSet = new DataSet();
            //SqlDataAdapter dataAdapter = new SqlDataAdapter(sql,DbHelper.connection);
            //try
            //{
            //    dataAdapter.Fill(dataSet, "UserType");
            //}
            //catch (Exception ex)
            //{
            //    MessageBox.Show(ex.Message);
            //    return;
            //}

            //cbxUserStyle.DataSource = dataSet.Tables[0];

            cbxUserStyle.SelectedIndex = 0;//默认选择第一项



        }
    }
}