奇怪的错误

计算记录条数时发生错误：Invalid attempt to Read when reader is closed.
读取列表时发生错误:Invalid attempt to FieldCount when reader is closed.        SqlCommand cmd = new SqlCommand();
        if (HttpContext.Current.Request.QueryString["Page"] != null)
        {
            lngPage = GetNum(HttpContext.Current.Request.QueryString["Page"].ToString(), 1);
        }
        else
        {
            lngPage = 1;
        }
        //计算总记录数
        try
        {
            cmd.CommandText = "Select Count(fdPID) as CID From tb_ClassPicINfo Where fdClass=" + intClassID;
            cmd.Connection = clsDatabase.getConn();
            if (cmd.ExecuteScalar() != null)
            {
                lngRsCount = Convert.ToInt64(cmd.ExecuteScalar());
                lngPgCount = GetPageCount(lngRsCount, intTop);
                if (lngPage > lngPgCount) lngPage = lngPgCount;
                strPageList = this.GetPageList(lngPage, lngPgCount, 5, "");
            }
            else
            {
                lngRsCount = 0;
            }
        }
        catch (Exception err1)
        {
            HttpContext.Current.Response.Write("计算记录条数时发生错误：" + err1.Message);
            AddErrorMessage(err1.Message, err1.Source, err1.TargetSite.Name.ToString());
        }
        finally
        {
            cmd.Dispose();
            clsDatabase.CloseDB();
        }
这里面没有用到SqlDataReader啊,是什么原因造成的???