DataTable dt = new DataTable();
                string sql = "select * from yh where 用户名='" + yhm.Text.Trim() + "'and 密码='" + mm.Text.Trim() + "'";
                SqlDataAdapter adp = new SqlDataAdapter(sql, myclass.opendata());
                adp. = sql;
                adp.Fill(dt);
//以上代码如何写成方法后，像下面一样做判断

                if (dt != null)
                {
                    if (dt.Rows.Count > 0)
                    {
                        main zt = new main();
                        zt.Show();
                        this.Hide();
                    }

       /// <summary>
        /// 生成DataTable对象
        /// </summary>
        /// <param name="cmd">命令对象</param>
        /// <returns>数据表对象</returns>
        public static DataTable CreateDataTable(SqlCommand cmd)
        {
            DataTable dt = new DataTable();
            SqlDataAdapter dataAdapter = new SqlDataAdapter(cmd);
            dataAdapter.Fill(dt);
            return dt;
        }

没有明白你说的什么，能说详细下吗

建议你去学习下sqlHelper这个静态类，里面都有很多的封装方法