asp.net AutoPostBack시 DropDownList 값 유지하기

if (ddl_Step != null && Request.Form[ddl_Step.UniqueID] != null && !Request.Form[ddl_Step.UniqueID].Equals(string.Empty))
{
ddl_Step.SelectedValue = Request.Form[ddl_Step.UniqueID].ToString();
}

if (ddl_Product != null && Request.Form[ddl_Product.UniqueID] != null && !Request.Form[ddl_Product.UniqueID].Equals(string.Empty))
{
ddl_Product.SelectedValue = Request.Form[ddl_Product.UniqueID].ToString();
}
if (lbActorName != null && Request.Form[lbActorName.UniqueID] != null && !Request.Form[lbActorName.UniqueID].Equals(string.Empty))
{
lbActorName.Text = Request.Form[lbActorName.UniqueID].ToString();
}

ddl_Step, ddl_Product DropDownList ID값
lbActorName textbox 값