@using CountryCityData

<div class="row cw-480">
    <div class="col-md-6">
        <label for="cbCountryName" class="mb-1">
            Country Name
        </label>
        <DxComboBox Data="@Countries"
                    TextFieldName="@nameof(Country.CountryName)"
                    Value="@CurrentCountry"
                    ValueChanged="@((Country country) => SelectedCountryChanged(country))"
                    AllowUserInput="true">
                    InputId="cbCountryName">
        </DxComboBox>
    </div>
    <div class="col-md-6">
        <label for="cbCityName" class="mb-1">
            City Name
        </label>
        <DxComboBox Data="@CurrentCountryCities"
                    TextFieldName="@nameof(City.CityName)"
                    @bind-Value="@CurrentCity"
                    AllowUserInput="true"
                    InputId="cbCityName" >
        </DxComboBox>
    </div>
</div>

@code {
    List<Country> Countries { get; set; } = CountryData.Countries;
    List<City> CurrentCountryCities { get; set; } = new List<City>();
    Country CurrentCountry { get; set; } = CountryData.Countries[1];
    City CurrentCity { get; set; } = CityData.Cities[4];

    protected override void OnInitialized() {
        base.OnInitialized();
        SelectedCountryChanged(CurrentCountry);
    }

    void SelectedCountryChanged(Country country) {
        CurrentCountry = country;
        CurrentCountryCities = CityData.Cities.FindAll(city => city.CountryId == CurrentCountry.Id);
        CurrentCity = CurrentCountryCities[0];
    }
}

namespace CountryCityData {
    public class City {
        public int Id { get; set; }
        public int CountryId { get; set; }
        public string CityName { get; set; }
    }

    public static class CityData {
        private static readonly Lazy<List<City>> cities = new Lazy<List<City>>(() => {
            return new List<City>() {
            new City() { Id = 0, CountryId = 0, CityName = "Washington" },
            new City() { Id = 1, CountryId = 0, CityName = "New York" },
            new City() { Id = 2, CountryId = 0, CityName = "Los Angeles" },
            new City() { Id = 3, CountryId = 1, CityName = "Berlin" },
            new City() { Id = 4, CountryId = 1, CityName = "Munich" },
            new City() { Id = 5, CountryId = 1, CityName = "Hamburg" },
            new City() { Id = 6, CountryId = 2, CityName = "Tokyo" },
            new City() { Id = 7, CountryId = 2, CityName = "Osaka" },
            new City() { Id = 8, CountryId = 2, CityName = "Yokohama" }
        };
        });
        public static List<City> Cities { get { return cities.Value; } }
    }
    public class Country {
        public int Id { get; set; }
        public string CountryName { get; set; }
    }

    public static class CountryData {
        private static readonly Lazy<List<Country>> countries = new Lazy<List<Country>>(() => {
            return new List<Country>()
                {
            new Country() { Id = 0, CountryName = "USA" },
            new Country() { Id = 1, CountryName = "Germany" },
            new Country() { Id = 2, CountryName = "Japan" }
        };
        });
        public static List<Country> Countries { get { return countries.Value; } }
    }
}

<DxComboBox Data="FirstDataSource"
            ValueFieldName="@nameof(MyModel.ID)"
            TextFieldName="@nameof(MyModel.Name)"
            Value="@FirstValue" 
            ValueChanged="@((int? value) => { FirstValue = value; keyValue++;})" />

<DxComboBox @key=@keyValue
            DataAsync="(cancellationToken) => LoadDataAsync(FirstValue, cancellationToken)"
            ValueFieldName="@nameof(MyModel.ID)"
            TextFieldName="@nameof(MyModel.Name)"
            @bind-Value="@SecondValue" />

@code {
   int keyValue;
   IEnumerable<MyModel> FirstDataSource { get; set; }
   int? FirstValue { get; set; }
   string? SecondValue { get; set; }
   public class MyModel {
       public int ID { get; set; }
       public string Name { get; set; }
   }
   protected override void OnInitialized() {
       FirstDataSource = Enumerable.Range(0, 10).Select(i => new MyModel() { ID = i, Name = $"Name {i}" });        
   }
   public async Task<IEnumerable<string>> LoadDataAsync(int? value, CancellationToken cancellationToken = default(CancellationToken)) {
       if(!value.HasValue)
           return Enumerable.Empty<string>();
       return FirstDataSource.Where(i => i.ID % value == 0).Select(i => i.Name);
   }
}