set below properties for your DropDownlist (
AutoPostBack
and
onselectedindexchanged
)
<asp:dropdownlist id="ddlCustomer" runat="server" autopostback="true" onselectedindexchanged="ddlCustomer_SelectedIndexChanged" xmlns:asp="#unknown"> </asp:dropdownlist>
the
onselectedindexchanged
event handler is as below
protected void ddlCustomer_SelectedIndexChanged(object sender, EventArgs e)
{
}