[求助]关于gridview中dropdownlist的问题

我在RowDataBound事件中填充每行ddl中的项,代码如下:if (e.Row.RowType != DataControlRowType.DataRow)
                return;
            GridViewRow gvr = e.Row;
            DropDownList ddl = (DropDownList)gvr.FindControl("ddlRooter");
            ListItem lirow = new ListItem("根分类", "0");
            ddl.Items.Add(lirow);
            int id = int.Parse(((HiddenField)gvr.FindControl("hfId")).Value);
            foreach (CategoryModel c in cm)
            {
                if (c.CategoryIn == 0 && c.CategoryId != id)
                {
                    lirow = new ListItem(c.CategoryName, c.CategoryId.ToString());
                    ddl.Items.Add(lirow);
                }
            }
            string CgyIn = ((HiddenField)gvr.FindControl("hfIn")).Value;
            for (int i = 0; i < ddl.Items.Count; i++)
            {
                if (ddl.Items[i].Value == CgyIn.ToString())
                    ddl.SelectedIndex = i;
            }这时候如果我要编辑某行的值,并且修改的ddl的选中项,在RowUpdating中获取选中项的值,代码如下:GridViewRow gvr = gvCategoryManager.Rows[rowIndex];
                int id = int.Parse(((HiddenField)gvr.FindControl("hfId")).Value);
                if (id == 1)
                {
                    ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(), "delCgySuccess", "<script>$().mjShow('系统默认分类不允许修改!')</script>", false);
                    return;
                }
                string name = TextHandler.FilteHTML(((TextBox)gvr.FindControl("tbName")).Text, 20);
                if (name.Length <= 0)
                {
                    ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(), "NewCgyRequired", "<script>$().mjShow('分类名称未填')</script>", false);
                    return;
                }
                DropDownList ddl = (DropDownList)gvr.FindControl("ddlRooter");
                int rooterId = int.Parse(ddl.SelectedItem.Value);
                CategoryModel updateCm = new CategoryModel(id, name, rooterId);
                Category cgy = new Category();
                if (cgy.UpdateCategory(updateCm) > 0)
                    ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(), "UpdateCgySuccess", "<script>$().mjShow('更新分类成功')</script>", false);
                else
                    ScriptManager.RegisterClientScriptBlock(this.Page, this.GetType(), "UpdateCgyFailed", "<script>$().mjShow('更新分类失败')</script>", false);
问题出现了:ddl的又变回默认的选项了!我调试了老半天发现在DataBind()方法中会重新执行一遍RowDataBound,于是我把它放进了if(!IsPostBack)代码块中.
此时,RowDataBound不触发了,但是ddl的值并没有如我所愿的成为选中项,而还是默认选项.我已经不知所措了,求各位达人帮忙解决,小弟不甚感激!