非常简单，直接给出代码：

数据库操作类

这个无需多言就是简单的包含了数据操作的内容，允许你在这一个类中写完关于本地数据库或者云数据库操作的逻辑，与登录逻辑分开哦。

注意，如果你的软件要给别人运行使用，你的数据库信息也可以存在授权文件中，以避免出现信息的泄露。如果你使用软件直接操作数据库可能存在危险，请注意连接数据库时对sql语句的检查。

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

namespace chuankouzhushou1
{
    internal class dataoperator
    {
        private static string connString = @"Data Source=DREAMER;Database=db_Mydb;User ID=sa;Pwd=yourpasswd;";

        public static SqlConnection connection = new SqlConnection(connString);

        public int ExecuteSQL(string sql)
        {
            SqlCommand cmd = new SqlCommand(sql, connection);
            if (connection.State == ConnectionState.Closed)
                connection.Open();
            int num = Convert.ToInt32(cmd.ExecuteScalar());        //执行查询
            connection.Close();
            return num;
        }
        public int ExecuteSQLResult(string sql)
        {
            SqlCommand cmd = new SqlCommand(sql, connection);
            if (connection.State == ConnectionState.Closed)
                connection.Open();
            int result = cmd.ExecuteNonQuery();
            connection.Close();
            return result;
        }
        public DataSet GetDataSet(string sql)
        {
            SqlDataAdapter sqlda = new SqlDataAdapter(sql, connection);    //指定要执行的SQL语句
            DataSet ds = new DataSet();
            sqlda.Fill(ds);
            return ds;
        }

        public SqlDataReader GetDataReader(string sql)
        {
            SqlCommand cmd = new SqlCommand(sql, connection);
            if (connection.State == ConnectionState.Closed)
                connection.Open();
            SqlDataReader dataReader = cmd.ExecuteReader();//添加参数CommandBehavior.CloseConnection就可以自动释放连接
            return dataReader;
        }
    }
}

输入的监测 

 private bool ValidateInput()
 {
     if (tx_ID.Text.Trim() == "")                            //登录账号
     {
         MessageBox.Show("请输入登录账号", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
         tx_ID.Focus();                                    //使登录账号文本框获得鼠标焦点
         return false;
     }
     else if (tx_ID.Text.Length>30)
     {
         MessageBox.Show("请输入正确的登录账号", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
         tx_ID.Focus();                                    //使登录账号文本框获得鼠标焦点
         return false;
     }
     else if (tx_ID.Text.Length > 5 && tx_PWD.Text.Trim() == "")   //密码
     {
         MessageBox.Show("请输入密码", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
         tx_PWD.Focus();                                    //使密码文本框获得鼠标焦点
         return false;
     }
     return true;
 }

 登录按钮（回车实现登录）

        private void loginbtn(object sender, EventArgs e)
        {
            if(ValidateInput())
            {
                int num= 0;
                string sql = "select count(*) from usertb where userid=" + int.Parse(tx_ID.Text.Trim()) + " and userpwd = '" + tx_PWD.Text.Trim() + "'";
                try
                {
                    num = dbop.ExecuteSQL(sql);
                }
                catch {
                    //MessageBox.Show(e.ToString());
                }   
                
                if (num == 1)//验证通过
                {
                    publicinfo.userid = int.Parse(tx_ID.Text.Trim());
                    // 执行新的SQL查询来获取userid，username和userlv
                    sql = "select userid, username, userlv from usertb where userid=" + publicinfo.userid;
                    SqlDataReader reader = dbop.GetDataReader(sql);
                    if (reader.Read())
                    {
                        publicinfo.userid = reader.GetInt32(0);
                        publicinfo.username = reader.GetString(1);
                        publicinfo.userlv = reader.GetInt32(2);
                    }
                    reader.Close();
                    mainfrm frm_Main = new mainfrm();
                    frm_Main.Show();
                    this.Visible = false;
                }
                else
                {
                    MessageBox.Show("登录失败！", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                }
            }
            
        }

        private void tx_ID_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (char.IsDigit(e.KeyChar) || (e.KeyChar == '\r') || (e.KeyChar == '\b'))
                e.Handled = false;
            else
                e.Handled = true;
        }