While User clicks on edit the Dropdownlist will appear and filled with available data.

I want to show selected value on the top.

i m getting the data in dropdownlist but not the selected one.plz tell me what to do so that i can get selected value on the top.

here is my code:-
<asp:TemplateField HeaderText="DepartmentName">
<ItemTemplate>
<asp:Label ID="lbldepartment" runat="server" Text='<%# Eval("DEPARTMENT_NAME")%>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<aspropDownList ID="dllDept" AutoPostBack="true" runat="server">
<asp:ListItem ></asp:ListItem>
</aspropDownList>
</EditItemTemplate>
</asp:TemplateField>

aspx.cs (By this i m geting the data in the dropdown)
protected void gvSupervisor_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
DropDownList ddldep = (DropDownList)e.Row.FindControl("dllDept");
Label lbdep = (Label)e.Row.FindControl("lbldepartment");
if (ddldep != null)
{
ddldep.DataSource = objSupervisor.GetDepartment();
ddldep.DataTextField = "DEPARTMENT_NAME";
ddldep.DataValueField = "DEPARTMENT_ID";

ddldep.DataBind();
ddldep.SelectedValue = lbdep.Text;

}
}