Why asp.net dropdown with autopostback doesn't keep my selected value

[EncacieveBync]

As advised here ASP.NET Response.Redirect(Request.RawUrl) doesn't work I did put autopostback = true on a dropdown list to reload my page.But after reloading it resets also the selected item to first one.How to keep my previous value before reloading the page then ? I thought autopostback would do that job ?