Use
Autopostback="true"
<asp:dropdownlist id="DropDownList1" runat="server" onselectedindexchanged="DropDownList1_SelectedIndexChanged" AutoPostBack="true" >
And you need to turn on the viewstate.
EnableViewState="true"
Also make sure the event name is correct. It should be
OnSelectedIndexChanged
not
SelectedIndexChanged
. Also check the event name matching on both codebehind & ASPX
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
}
Also another issue & solution(Not sure your issue is same as this one)
Detect DropDownList SelectedIndexChanged on Page Load[
^]
Gotcha I found that link
ASP.NET: Error: DropDownList OnSelectedIndexChanged Event Not Firing[
^]