Hi,
it can be accomplished using jquery.
ProductList = new ProductsBLL().GetAllProductsList();
this.ddl.DataValueField = "Id";
this.ddl.DataTextField = "ProductName";
this.ddl.DataSource = ProductList;
this.ddl.DataBind();
and the jquery
$('#Main_ddl').change(function () {
alert($('#Main_ddl').val());
});
now, where I have used alert, you can use ajax call to go with your further code or to build a logic.
thanks