
protected void ddl1_Load(object sender, EventArgs e)
{
DropDownList ddl=(DropDownList)sender;
ddl.DataSource = (...call yur values here...)
ddl.DataValueField = "Column name ";
ddl.DataBind();
}
<asp:DropDownList ID="ddlOrderType" runat="server" Onload="ddl1_load()">
</asp:DropDownList>