Populate (Bind) DropDownList based on another in ASP.Net MVC

Hello,

i am trying to create two dropdownlist one is having list of designations and another is having a list of names on the basis of designations

i am using only one database from this so i am not able to get the distinct values in dropdownlist 1 with designation name and on the basis of that usernames in other dropdownlist.

please help

@model WebApplication2.Models.CascadingModel
@{
    ViewBag.Title = "Index";
}

@using (Html.BeginForm("Index", "Register", FormMethod.Post))
{
<div class="form-horizontal">
    <div class="panel panel-primary" style="width:620px;margin-left:auto;margin-right:auto;margin-top:30px">
        <div class="panel-heading">
            <h4>User Registration</h4>
        </div>
        <div class="panel-body">
            <div class="form-group">
                @Html.Label("Select Designation", htmlAttributes: new { @class = "control-label col-md-4" })
                <div class="col-md-8">
                    @Html.DropDownListFor(model=>model.Designationname, new SelectList(ViewBag.desingations, "Designation","Designation"), htmlAttributes: new { onchange = "documents.fors[0].submit();", @class = "form-control" })
                </div>
            </div>
            
        </div>
    </div>
</div>
}

using Microsoft.Ajax.Utilities;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using WebApplication2.Models;

namespace WebApplication2.Controllers
{
    public class RegisterController : Controller
    {
        // GET: Register
        public ActionResult Index()
        {
            ApplicationDbContext db = new ApplicationDbContext();
            CascadingModel model = new CascadingModel();
            //var query = model.Designation.DistinctBy(x => new { x.Text, x.Value });
            List<SelectListItem> desingations = new List<SelectListItem>();
            desingations = (from c in db.Deisngations
                            select new SelectListItem { Text = c.Designation, Value = c.DesigId.ToString() }).Distinct().ToList();
            //var uniquerdesig = desingations.Distinct();

            ViewBag.designation = desingations;
            return View();
        }
    }
}

namespace WebApplication2.Models
{
    public class CascadingModel
    {
        public CascadingModel()
        {
            this.Designation = new List<SelectListItem>();
            this.Username = new List<SelectListItem>();
            this.Email = new List<SelectListItem>();
        }
        public List<SelectListItem> Designation { get; set; }
        public List<SelectListItem> Username { get; set; }
        public List<SelectListItem> Email { get; set; }

        public string Designationname { get; set; }
        public string user { get; set; }
        public string EmilId { get; set; }
    }
}