using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using MySql.Data.MySqlClient;

namespace Db
{
    public class SqlHelper
    {
        private MySqlCommand cmd;//
 
執行一條sql語句。
        private MySqlConnection con;
        private MySqlDataReader reader = null;
        MySqlDataAdapter msda;//Represents a set of data commands and a database connection that are used to fill a data set and update a MySQL database. 

        //連線字串
        private static string connStr = ConfigurationManager.ConnectionStrings["
 
connStr"].ConnectionString;

        //查詢與ataGridView控制元件配套
        public DataTable ExecuteQuery_DataTable(string sqlStr)
        {
            //查
            DataTable dt = new DataTable();//用於ataGridView控制元件
            con = new MySqlConnection(connStr);
            con.Open();//開啟連結,可以省略，建議寫上
            cmd = new
 
 MySqlCommand(sqlStr, con);
            try
            {
                cmd.CommandType = CommandType.Text;
                msda = new MySqlDataAdapter(cmd);
                msda.Fill(dt);
            }
            catch (Exception ex) {}
            return dt;
        }

        //查詢使用reader，一次讀一條，類似於c語言讀檔案。
        public void ExecuteQuery(string sqlStr)
        {
            //查
            con = new MySqlConnection(connStr);
            con.Open();//開啟連結,可以省略，建議寫上
            cmd = new MySqlCommand(sqlStr, con);
            reader = cmd.ExecuteReader();
                //while (reader.Read())
                //{
                //    string str="ID=" + reader[0].ToString() + " ,TITLE=" + reader[1].ToString() + " ,KEYWORD=" +
                //    reader[2].ToString() + " ,CONTENT=" + reader[3].ToString() + ".";
                //}

        }
        public int ExecuteUpdate(string sqlStr)
        {
            //增刪改
            MySqlCommand cmd;
            MySqlConnection con;
            con = new MySqlConnection(connStr);
            con.Open();
            cmd = new MySqlCommand(sqlStr,con);
            cmd.CommandType = CommandType.Text;
            int iud = 0;
            iud = cmd.ExecuteNonQuery();
            con.Close();
            return iud;
        }
        public void allClose()
        {
            con.Close();
        }

        public MySqlDataReader getReader()
        {
            return reader;
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using Db;
using MySql.Data.MySqlClient;

namespace WindowsFormsAppCS
{
    public partial class Form1 : Form
    {

        private Form2 anotherForm;
        private SqlHelper sqlHelper = new SqlHelper();
        public Form1()
        {
            InitializeComponent();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            //資料清空
            userid_input.Clear();
            userpassword_input.Clear();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            //接收資訊
            string userName = userid_input.Text.Trim();
            string userPass = userpassword_input.Text.Trim();
            //判斷是否為空
            if(string.IsNullOrEmpty(userName))
            {
                MessageBox.Show("使用者名稱不能為空");
                userid_input.Focus();
                return;
            }
            if (string.IsNullOrEmpty(userPass))
            {
                MessageBox.Show("密碼不能為空");
                userid_input.Focus();
                return;
            }

            //登入檢查
            string sql = "select * from users where userid='" + userName + "' and userpass='" + userPass + "'";
            sqlHelper.ExecuteQuery(sql);
            if(sqlHelper.getReader().Read())
            {
                sqlHelper.allClose();
                anotherForm = new Form2();
                this.Hide();
                anotherForm.ShowDialog();
                Application.ExitThread();
            }
            else
            {
                MessageBox.Show("使用者名稱密碼錯誤");
                sqlHelper.allClose();
                return;
            }
        }
    }
}