public ActionResult Cascading()
        {
            //Creating object of Service class Car. We have a separate service class to query to DB.
            Car carService = new Car();
            //Creating object of ViewModel which will pass as model to the view.
            CarViewModel model = new CarViewModel();
            //Creating IEnumerable list of SelectedListItem.
            List<SelectListItem> listcompanies = new List<SelectListItem>();
            //Creating List of CarCompany object which will hold the list of CarCompany objects from DB.
            List<CarCompany> carCompanies = new List<CarCompany>();
            //We are populating list by calling GetCompanyListFromDB method of service, which will bring 
            //all the record of CarCompany table from db
            carCompanies =  carService.GetCompanyListFromDB();
            //We are adding Select as first item of out Selected List
            listcompanies.Add(new SelectListItem { Text = "Select", Value = "Select", Selected = true });
            //We are iterating through the CarCompanyList and are using Company name and Id to create SelectedList.
            foreach (CarCompany item in carCompanies)
            {
                listcompanies.Add(new SelectListItem { Text = item.companyName,Value=item.Id.ToString() });
            }
            //Selected List is assigned to model's property.
            model.listCarCompany = listcompanies;
            //View is returned with model as parameter.
            return View(model);
        }

[HttpPost]
        //Gets the Cars based on the company selected in first Dropdown.
        //Method accepts selectedValue as parameter. This is basically the selected value of first dropdown.
        public ActionResult GetCarsBycompany(string selectedValue)
        {
            //Created object of service class which holds the method that queries database.
            Car carService = new Car();
            //Created a list of string to hold car names from Database.
            List<string> carList = new List<string>();
            //Populating the list by calling GetCarsByCompanyId method of service class.
            carList = carService.GetCarsByCompanyId(Convert.ToInt32(selectedValue));
            //Returning the list using jSon.
            return Json(new { CarList = carList });
        }

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using CascadingDropdowns.Models;

namespace CascadingDropdowns.Service
{
    public class Car
    {
        //Gets the CarCompany List from DB.
        public List<CarCompany> GetCompanyListFromDB()
        {
            //Created DataContext to query DB.
            using(CascadingDropdownEntities dataContext = new CascadingDropdownEntities())
            {
                //returns all the records present in CarCompany table in list format.
                return dataContext.CarCompanies.ToList();
            }
        }

        //Gets the CarModel list based on CompanyId which is foreign key in Carmodel table.
        public List<string> GetCarsByCompanyId(int companyId)
        {
            //Created DataContext to query DB.
            using (CascadingDropdownEntities dataContext = new CascadingDropdownEntities())
            {
                //returns all the records from table based on companyId in list format.
                return dataContext.CarModels.Where(query => query.CarCompanyId == companyId).Select(q => q.CarName).ToList();
            }
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.Mvc;
using CascadingDropdowns.Models;

namespace CascadingDropdowns.ViewModel
{
    //This object of this model is passed as parameter to view.
    public class CarViewModel
    {
        //List to hold the company names which is passed as model property.
        //This list is used to populate the first dropdown.
        public List<SelectListItem> listCarCompany { get; set; }
    }
}

@model CascadingDropdowns.ViewModel.CarViewModel
@{
    Layout = "../Shared/_Layout.cshtml";
}


<div>
@Html.DropDownListFor(model => model.listCarCompany, Model.listCarCompany)
<select id="Cars" style="width:100px;">
</select>
</div>

<script type="text/javascript">
    $("#listCarCompany").change(function () {
        var selectedValue = $(this).val();
        $.ajax({
            url: '@Url.Action("GetCarsBycompany", "Car")',
            type: 'POST',
            data: { "selectedValue": selectedValue },
            dataType: 'json',
            success: function (response) {
                var items = "";
                $.each(response.CarList, function (i, item) {
                    items += "<option value=\"" + item + "\">" + item + "</option>";
                });
                $("#Cars").html(items);
            },
            error: function (error) {
            }

        });
    });
</script>