连接时到是da.fill（ds）错误！！

    public class dbbase
    {
        //static readonly string connstr = ConfigurationSettings.AppSettings["connstr"].ToString();
        //static readonly string connstr = "server=.;database=student;uid=sa;pwd=";
        //static readonly string connstr = System.Configuration.ConfigurationManager.AppSettings["connstr"].ToString();
        static readonly string connstr = "Data Source=(local);Initial Catalog=|DataDirectory|GRADE.mdf;Integrated Security=SSPI";
        public dbbase()
        {
            //
            // TODO: 在此处添加构造函数逻辑
            //
        }

        public static DataTable getDataTable(string sqlstr)
        {
            SqlConnection conn = new SqlConnection(connstr);
            SqlDataAdapter da = new SqlDataAdapter(sqlstr,conn);
            DataSet ds =new DataSet();
            da.Fill(ds);
            return ds.Tables[0];
        }

        public static void ExecuteSql(string sqlstr)
        {
            SqlConnection conn = new SqlConnection(connstr);
            SqlCommand cmd = new SqlCommand(sqlstr,conn);
            conn.Open();
            cmd.ExecuteNonQuery();
            conn.Close();
        }
        public static void UpdateDt(DataTable changedDt, string sqlstr)
        {
            SqlConnection conn = new SqlConnection(connstr);
            SqlDataAdapter da = new SqlDataAdapter(sqlstr, conn);
            SqlCommandBuilder sqlCmdBuilder = new SqlCommandBuilder(da);
            da.Update(changedDt);
            changedDt.AcceptChanges();
        }
        public static DataSet getDataSet(string sql)
        {
            SqlConnection conn = new SqlConnection(connstr);
            SqlDataAdapter da = new SqlDataAdapter(sql, conn);
            DataSet ds = new DataSet();
            da.Fill(ds);
            return ds;
        }
    }
}