Repeater嵌套dropdownlist控件

<asp:repeater id="Repeater1" runat="server"onitemdatabound="Repeater1_ItemDataBound">
            <itemtemplate>
            <div>
                <asp:dropdownlist id="drpUserList" autopostback="true"onselectedindexchanged="myDrop_SelectedIndexChanged" runat="server"></asp:dropdownlist>
            </div>
            </itemtemplate>
        </asp:repeater>

protected void Repeater1_ItemDataBound(object sender, RepeaterItemEventArgs e)
    {
        if (e.Item.ItemType ==ListItemType.Item||e.Item.ItemType==ListItemType.AlternatingItem||e.Item.ItemType==ListItemType.EditItem)
        {
            DropDownList myDrop = e.Item.FindControl("drpUserList") as DropDownList;
            if (myDrop.Items.Count == 0)
            {
                myDrop.Items.Add(new ListItem("a", "1"));
                myDrop.Items.Add(new ListItem("b", "2"));
                myDrop.DataBind();
            }
        }
    }

    protected void myDrop_SelectedIndexChanged(object sender,EventArgs e)
    {
        Response.Write(((DropDownList)sender).SelectedIndex.ToString());
    }