where to write coding of dropdownlist selected index change in mvc razor application

@Html.DropDownList(
    "PropertyTypePropertyTypeId",
     ViewData["Categories"], // it need to be casted  i think 
    null,
    new { onchange = "document.location.href = '/Controller/MyAction?country=' + this.options[this.selectedIndex].value;" })

public ActionResult MyAction(string country ) 
            {
                 // return the adequate view depending on the country argument 
                  .....
            }

//create a method to get Dropdown values 

public ViewResult Index()
        {
            var categories = from c in db.category_master select c;
            ViewData["Categories"] = new SelectList(categories, "ID", "Name");
            return View(db.akshay_category_master.ToList());

        }

// In view 

@Html.DropDownList("Categories", ViewBag.applicationList as SelectList, "Select", new { onchange = "javascript:sendParam();" })

 //Javascript Function 

<script type="text/javascript">
    function sendParam() {

        window.location = "Category/CategoryDetails/" + jQuery("#Categories").val();

    }

  // that Method in Controller

  public ViewResult CategoryDetails(int id)
        {

            var Result = from C in db.category_master
                         where C.id == id
                         select C;
            return View("CategoryDetails", Result.ToList());
          
        }