string username = textBox1.Text;
            string password = textBox2.Text;
            using (SqlCommand cmd = conn.CreateCommand())
            {
                string cmdStr = "SELECT * FROM User_T　where UserName= '"+username+"'";
                SqlCommand sqlCom = new SqlCommand(cmdStr, conn);//定义查询命令
                SqlDataReader reader = sqlCom.ExecuteReader();//执行查询
                if (reader.Read())
                {
                    //用户名存在
                    string dbpassword = reader.GetString(reader.GetOrdinal("Password"));
                    if (password == dbpassword)//比较数据
                    {
                        MessageBox.Show("登陆成功");
                    }
                    else
                    {
                        MessageBox.Show("密码错误，登陆失败");
                    }
                }
                else
                {
                    MessageBox.Show("用户名错误");
                }    
            }