在不工作的中继器中设置Dropdownlist的Selectedindex/selectedvalue

我有一个中继器里面有一个下拉列表。我在itembound事件中设置这个列表的数据源,然后设置selectedindex。当我调试时,selectedindex被设置,但是当页面加载完所有项目时,默认项目被选中。

这是我的密码:

 protected void Repeater1_ItemDataBound(object sender, RepeaterItemEventArgs e)
    {
        var ddl = (DropDownList)e.Item.FindControl("DataFeedItems");
        ddl.DataSource = FilterDropDownData();
        ddl.DataTextField = "ColumnName";
        ddl.DataValueField = "ColumnName";
        ddl.DataBind();
        ddl.SelectedValue = "Select";
        ddl.SelectedIndex = 28;
    }

  protected DataTable FilterDropDownData()
    {
        var importedFeedColums = I make the table here;
        DataRow newRow = importedFeedColums.NewRow();
        newRow[0] = "Selecteren";
        importedFeedColums.Rows.Add(newRow);
        return importedFeedColums;
    }

    protected void DataFeedItems_DataBound(object sender, System.EventArgs e)
    {
        var ddl = (DropDownList) (sender);
        ddl.SelectedValue = "Selecteren";
        ddl.Items[28].Selected = true;

    }

 <asp:Repeater ID="Repeater1" runat="server" 
            onitemdatabound="Repeater1_ItemDataBound">
            <ItemTemplate>
                <li>
                <label><%# DataBinder.Eval(Container.DataItem, "ColumnName") %></label>                       
                    <asp:DropDownList ID="DataFeedItems" ClientIDMode="Static" runat="server" DataSource='<%# FilterDropDownData() %>'
                        DataTextField="ColumnName" DataValueField="ColumnName"   OnDataBound="DataFeedItems_DataBound" >
                    </asp:DropDownList>
                    <input id="Hidden1" runat="server" clientidmode="Static" type="hidden" value='<%# DataBinder.Eval(Container.DataItem, "ColumnName") %>' />
                </li>
            </ItemTemplate>
        </asp:Repeater>

当我发布表单时,我可以得到每个DropDownList的selectedvalue和文本。我做错什么了