PersonForm.razor

@code {
    [Parameter] public Person Person { get; set; }
    [Parameter] public string ButtonText { get; set; } = "Save Person";
    [Parameter] public EventCallback OnValidSubmit { get; set; }
    private List<Country> Countries = new List<Country>();
    private List<State> States = new List<State>();
    private int countryId = 0;

    protected override async Task OnInitializedAsync()
    {

        if (Person.State != null)
        {
            countryId = Person.State.CountryId;
            await LoadStates(countryId);
        }

        Countries = await http.GetJsonAsync<List<Country>>("api/countries");
    }

    private async Task CountryHasChanged(int value)
    {
        Person.StateId = 0;
        countryId = value;

        if (value == 0)
        {
            States.Clear();
        }
        else
        {
            await LoadStates(value);
        }
    }

    private async Task LoadStates(int countryId)
    {
        States = await http.GetJsonAsync<List<State>>($"api/countries/{countryId}/states");
    }
}