版主在么,做完登录界面后,程序崩溃,显示数据库登录失败: public static  DataSet ds = new DataSet();
        public static string connstring = "server=.;database=ssxt;uid=sa;pwd=123456;";
        //执行select命令,并返回结果写入数据集的表中
        public bool  Select(string cmdstr, string tablename)
        {
               SqlConnection  conn = new SqlConnection(connstring);
                conn.Open();
                SqlCommand cmd = new SqlCommand(cmdstr, conn);
                cmd.ExecuteNonQuery();
                SqlDataAdapter sda = new SqlDataAdapter(cmd);
                sda.Fill(ds, tablename);
                conn.Close();
                return true;
        }
        public void Insert(string cmdstr)
        {
            SqlConnection conn = new SqlConnection(connstring);
            conn.Open();
            SqlCommand cmd = new SqlCommand(cmdstr, conn);
            cmd.ExecuteNonQuery();
            conn.Close();
        }
        public void Update(string cmdstr)
        {
            SqlConnection conn = new SqlConnection(connstring);
            conn.Open();
            SqlCommand cmd = new SqlCommand(cmdstr, conn);
            cmd.ExecuteNonQuery();
            conn.Close();
        }
        public void Delete(string cmdstr)
        {
            SqlConnection conn = new SqlConnection(connstring);
            conn.Open();
            SqlCommand cmd = new SqlCommand(cmdstr, conn);
            cmd.ExecuteNonQuery();
            conn.Close();
这个是我编写的类,运行后出现:  这个是我的按钮代码: sql login = new sql();
        private void button1_Click(object sender, EventArgs e)
        {
            string strcmd;
            if (txtloginname.Text != "" && txtpassword.Text != "")
            {
                strcmd = "select * from ygb where yg_id=" + txtloginname.Text + " and mm='" + txtpassword.Text + "'";
                login.Select(strcmd, "login");
                if (sql.ds.Tables["login"].Rows.Count != 0)
                {
                    Form GLYJM = new frmlogin();
                    GLYJM.Show();
                    this.Hide();
                }
                else
                    MessageBox.Show("用户名或密码错误!");
            }