Cascading @Html.DropDownList

H All,

I had no experience in html/asp code.

Now, I'm trying to cascade 2 dropdown list. First of all, I prepare the Model

    #region Category
    public class Category
    {
        [Key]
        public int CategoryID {
            get; set;
        }

		public string CategoryName {
            get; set;
        }
    }
    #endregion


    #region Category_Sub
    public class Category_Sub
    {
        [Key]
        public int SubCategoryID {
            get; set;
        }

        public int CategoryID {
            get; set;
        }

        public string SubCategoryName {
            get; set;
        }

    }
    #endregion

and for Controller

        private readonly Project _project;

        public ProjectController(Project project)
        {
            _project = project;
        }

        // GET: /<controller>/
        public IActionResult Index()  {
            ViewData ["Message"] = "Project";

            List<Category> lst_Category = _project.Category.FromSql ("[dbo].[Select_Category]").ToList ( );
            lst_Category.Insert (0, new Category { CategoryID = 0, CategoryName = "--- Select ---" });

            List<Category_Sub> lst_Category_Sub = _project.Category_Sub.FromSql("[dbo].[Select_Category_Sub]").ToList ( );
            lst_Category_Sub.Insert (0, new Category_Sub { CategoryID = 0, SubCategoryName = "--- Select ---" });

            ViewBag.Category = lst_Category;
            ViewBag.Category_Sub = lst_Category_Sub;
            return View();
        }

and then, my questions are

1. how to bind those data from 'Category' list and 'Category_Sub' list into @Html.DropDownList ? (controller to view)
2. how to interact between each @HtmlDropDownList 'Category' and 'Category_Sub' ?

Thank in Adv

D