Dropdown event does not fire


I am having a DropdownLIst in the HeaderTemplate of the Grid I have written some server side code on selectedINdexChanged event of this dropdown But this event never fires. I have also Enabled the ViewState of dropdown and the Page to true Ant ide what must be the Problem

I have solved this problem in my environment.... Please check out below code..

This is my gridview in aspx page.

 <asp:GridView ID="grvGrid" runat="server" AllowPaging="True"
            AllowSorting="True" AutoGenerateColumns="False" DataKeyNames="CustomerID" OnRowDataBound="grvGrid_RowDataBound">
                <asp:BoundField DataField="CustomerID" HeaderText="CustomerID" ReadOnly="True"
                    SortExpression="CustomerID" />
                <asp:BoundField DataField="CompanyName" HeaderText="CompanyName"
                    SortExpression="CompanyName" />
                        <%--<asp:Label ID="lblMon1" runat="server"></asp:Label>--%>
                        <asp:DropDownList id="ddlMon" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlMon_SelecdtedIndexChanges">
                            <asp:ListItem Text="1" Value="1" Selected="True">1</asp:ListItem>
                            <asp:ListItem Text="2" Value="2">2</asp:ListItem>
                            <asp:ListItem Text="3" Value="3">3</asp:ListItem>
                        <asp:Label ID="lblblbl" runat="server"></asp:Label>

Get Grid View Bind in pageload event

protected void Page_Load(object sender, EventArgs e)
        if (!Page.IsPostBack)
            grvGrid.DataSource = dstable;

Find DropDown control and bind it's event

protected void grvGrid_RowDataBound(object sender, GridViewRowEventArgs e)
        if (Page.IsPostBack)
            if (e.Row.RowType == DataControlRowType.Header)
                DropDownList ddlmon = e.Row.FindControl("ddlMon") as DropDownList;
                ddlmon.SelectedIndexChanged += new EventHandler(ddlMon_SelecdtedIndexChanges);

DropDown SelectedIndex Changes Event

protected void ddlMon_SelecdtedIndexChanges(object sender, EventArgs e)
        // Your Code paste here