string[] groupSub = groupList.Split(',');
        sql = string.Empty;
        for (int i = 0; i < groupSub.Length; i++)
        {
            sql = "select id,user_truename from l_user where id='" + Convert.ToInt32(groupSub[i].ToString()) + "'";   //有问题
            table = DbHelperSQL.Query(sql).Tables[0];
            dlSelectUser.DataSource = table;
            dlSelectUser.DataBind();
        }

string[] groupSub = groupList.Split(',');
            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            for (int i = 0; i < groupSub.Length; i++)
            {
                int n = 0;
                if (int.TryParse(groupSub[i], out n))
                {
                    sb.Append(groupSub[i] + ",");
                }
               
            }
            if (sb.Length > 0)
            {
                sb.Remove(sb.Length - 1, 1);
                sql = "select id,user_truename from l_user where id IN(" + sb.Length() + ")";
                table = DbHelperSQL.Query(sql).Tables[0];
                dlSelectUser.DataSource = table;
                dlSelectUser.DataBind();
            }

------解决方案--------------------
dlSelectUser.DataSource = table;
           dlSelectUser.DataBind();
放在循环外面

------解决方案--------------------
C# code

 string[] groupSub = groupList.Split(',');
        sql = string.Empty;
        DataTable table =new DataTable();
        for (int i = 0; i < groupSub.Length; i++)
        {
            sql = "select id,user_truename from l_user where id='" + groupSub[i].ToString() + "'";   //有问题
            table = DbHelperSQL.Query(sql).Tables[0];
            ListItem lt = new ListItem();
            lt.Text = table.Rows[0]["user_truename"].ToString();
            lt.id = table.Rows[0]["id"].ToString();
            dlSelectUser.Items.Add(lt);

        }
        dlSelectUser.DataBind();

---这样做要和数据访问 好多次！
建议， select id,user_truename from l_user where id IN()
一次读出！在绑定！

------解决方案--------------------
C# code

string[] groupSub = groupList.Split(',');
            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            for (int i = 0; i < groupSub.Length; i++)
            {
                int n = 0;
                if (int.TryParse(groupSub[i], out n))
                {
                    sb.Append(groupSub[i] + ",");
                }
               
            }
            if (sb.Length > 0)
            {
                sb.Remove(sb.Length - 1, 1);
                sql = "select id,user_truename from l_user where id IN(" + sb.Length() + ")";
                table = DbHelperSQL.Query(sql).Tables[0];
                dlSelectUser.DataSource = table;
                dlSelectUser.DataBind();
            }

这个是一次，读出，直接绑定就行了！

------解决方案--------------------
学习中

------解决方案--------------------