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Configure the Cascading DropDownList

The cascading DropDownList is a series of DropDownList, where the value of one DropDownList depends upon another’s value. This can be configured by using the change event of the parent DropDownList. Within that change event handler, data has to be loaded to the child DropDownList based on the selected value of the parent DropDownList.

The following example, shows the cascade behavior of country, state, and city DropDownList. Here, the dataBind method is used to reflect the property changes immediately to the DropDownList.

razor
cascading.cs
Cities.cs
State.cs
Country.cs
<div class="padding-top">

    @Html.EJS().DropDownList("country").Placeholder("Select a country").PopupHeight(@ViewBag.popupHeight).Change("countrychange").DataSource((IEnumerable<Object>)ViewBag.country).Fields(new Syncfusion.EJ2.DropDowns.DropDownListFieldSettings { Text = "CountryName", Value = "CountryId" }).Render()
</div>
<div class="padding-top">

    @Html.EJS().DropDownList("state").Placeholder("Select a state").Enabled(false).PopupHeight(ViewBag.popupHeight).Change("statechange").DataSource((IEnumerable<Object>)ViewBag.state).Fields(new Syncfusion.EJ2.DropDowns.DropDownListFieldSettings { Text = "StateName", Value = "StateId" }).Render()
</div>
<div class="padding-top">

    @Html.EJS().DropDownList("city").Placeholder("Select a city").Enabled(false).PopupHeight(ViewBag.popupHeight).DataSource((IEnumerable<Object>)ViewBag.cities).Fields(new Syncfusion.EJ2.DropDowns.DropDownListFieldSettings { Text = "CityName", Value = "CityId" }).Render()
</div>

<script type="text/javascript">
    function countrychange() {
        // disable the state DropDownList
        var countryObj = document.getElementById('country').ej2_instances[0];
        var state = document.getElementById('state').ej2_instances[0];
        var city = document.getElementById('city').ej2_instances[0];
        state.enabled = true;
        //frame the query based on selected value in country DropDownList.
        var tempQuery = new ej.data.Query().where('CountryId', 'equal', countryObj.value);
        // set the framed query based on selected value in country DropDownList.
        state.query = tempQuery;
        // set null value to state DropDownList text property
        state.text = null;
        //  bind the property changes to state DropDownList
        state.dataBind();
        // set null value to city DropDownList text property
        city.text = null;
        // disable the city DropDownList
        city.enabled = false;
        // bind the property changes to City DropDownList
        city.dataBind();
    }
    function statechange() {
        var stateObj = document.getElementById('state').ej2_instances[0];
        var city = document.getElementById('city').ej2_instances[0];
        city.enabled = true;
        //Query the data source based on state DropDownList selected value
        var tempQuery = new ej.data.Query().where('StateId', 'equal', stateObj.value);
        //set the framed query based on selected value in city DropDownList.
        city.query = tempQuery;
        //clear the existing selection
        city.text = null;
        //bind the property change to city DropDownList
        city.dataBind();

    }
</script>
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using WebApplication1.Models;

namespace WebApplication1.Controllers
{
    public class DropDownListController : Controller
    {
        public ActionResult cascading()
        {
            ViewBag.state = new State().StateList();
            ViewBag.country = new Country().CountryList();
            ViewBag.cities = new Cities().CitiesList();
            ViewBag.popupHeight = "auto";
            return View();
        }
    }
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace WebApplication1.Models
{
    public class Cities
    {
        public string CityName { get; set; }
        public string StateId { get; set; }
        public int CityId { get; set; }

        public List<Cities> CitiesList()
        {
            List<Cities> cities = new List<Cities>();
            cities.Add(new Cities() { CityName = "Aberdeen", StateId = "103", CityId = 207 });
            cities.Add(new Cities() { CityName = "Alexandria", StateId = "102", CityId = 204 });
            cities.Add(new Cities() { CityName = "Albany", StateId = "101", CityId = 201 });
            cities.Add(new Cities() { CityName = "Beacon ", StateId = "101", CityId = 202 });
            cities.Add(new Cities() { CityName = "Brisbane ", StateId = "104", CityId = 211 });
            cities.Add(new Cities() { CityName = "Cairns", StateId = "104", CityId = 212 });
            cities.Add(new Cities() { CityName = "Colville ", StateId = "103", CityId = 208 });
            cities.Add(new Cities() { CityName = "Devonport", StateId = "105", CityId = 215 });
            cities.Add(new Cities() { CityName = "Emporia", StateId = "102", CityId = 206 });
            cities.Add(new Cities() { CityName = "Geelong", StateId = "106", CityId = 218 });
            cities.Add(new Cities() { CityName = "Hampton ", StateId = "102", CityId = 205 });
            cities.Add(new Cities() { CityName = "Healesville ", StateId = "106", CityId = 217 });
            cities.Add(new Cities() { CityName = "Hobart", StateId = "105", CityId = 213 });
            cities.Add(new Cities() { CityName = "Launceston ", StateId = "105", CityId = 214 });
            cities.Add(new Cities() { CityName = "Lockport", StateId = "101", CityId = 203 });
            cities.Add(new Cities() { CityName = "Melbourne", StateId = "106", CityId = 216 });
            cities.Add(new Cities() { CityName = "Pasco", StateId = "103", CityId = 209 });
            cities.Add(new Cities() { CityName = "Townsville", StateId = "104", CityId = 210 });
            return cities;
        }
    }
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace WebApplication1.Models
{
    public class State
    {
        public string StateName { get; set; }
        public string CountryId { get; set; }
        public string StateId { get; set; }
        public List<State> StateList()
        {
            List<State> state = new List<State>();
            state.Add(new State() { StateName = "New York", CountryId = "1", StateId = "101" });
            state.Add(new State() { StateName = "Queensland", CountryId = "2", StateId = "104" });
            state.Add(new State() { StateName = "Tasmania ", CountryId = "2", StateId = "105" });
            state.Add(new State() { StateName = "Victoria", CountryId = "2", StateId = "106" });
            state.Add(new State() { StateName = "Virginia ", CountryId = "1", StateId = "102" });
            state.Add(new State() { StateName = "Washington", CountryId = "1", StateId = "103" });
            return state;
        }
    }
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace WebApplication1.Models
{
    public class Country
    {
        public string CountryName { get; set; }
        public string CountryId { get; set; }

        public List<Country> CountryList()
        {
            List<Country> country = new List<Country>();
            country.Add(new Country() { CountryName = "Australia", CountryId = "2" });
            country.Add(new Country() { CountryName = "United States", CountryId = "1" });
            return country;
        }
    }
}