Controller//
ViewBag.ddlprincepal = new SelectList(objentity.ddlPrincipal(), "PrincipalID", "PrincipalName");
ViewBag.ddlprincepalselected = new SelectListItem { Text = inventoryDetailobj[0].PrincipalName, Value = (inventoryDetailobj[0].PrincipalID).ToString(),Selected=true };
View
//
@Html.DropDownList("ddlprincepal",ViewBag.ddlprincepal as SelectList ,ViewBag.ddlprincepalselected as string , new { @id = "ddlprincepal" })