how to get data from npgsql to dropdownlist c#

Question

<asp:dropdownlist ID="EndtypeDL" CssClass="form-control" AutoPostBack="true" runat="server" OnSelectedIndexChanged="EndtypeDL_SelectedIndexChanged">

</asp:dropdownlist>

 

var connString = WebConfigurationManager.AppSettings["ConnectionDB"];

var conDrop = new NpgsqlConnection(connString);

 

how to next? 

 thank you