Multi select dropdown select all on load with OnChange

Radzen.Blazor Components
1

I am attempting to get both checkbox lists to select all on load.
I have had issues trying to follow the example which appears to use @bind-Value and OnChange but I was not able to get it to work (and from what I understand it should not work)
Open to suggestions on how best to achieve preloading checkboxes with all items selected.

I am going to attempt to just use @bind-value with the settings below.
I believe the callback always has to have the signature Onchaged(object obj,string name) or it never gets called is that correct?

View

@page "/Counter"
@using Microsoft.Extensions.Configuration
@using VibeDashboardsDAL;
@using System.Data;
@inject IConfiguration Configuration
<style>
    .ui-multiselect-item{
        font-size:12px;
        font-weight:bold
    }
</style>
<EditForm Model="@formname">
    <div class="row">
        <div class="col-md-2 ">
            <div>Select Facilities</div>
            <RadzenListBox  TValue="object"
                           AllowFiltering="false"
                           FilterCaseSensitivity="FilterCaseSensitivity.CaseInsensitive"
                           Change=@(args => OnChange(args, "FacilityList"))
                           Multiple="true"
                           Data=@Facilities
                           TextProperty="FacilityName" ValueProperty="FacilityID"
                           Placeholder="Select All"
                           
                           Style="max-height:100px;overflow:scroll;width:160px" />
        </div>
        <div class="col-md-2">
            <div>Select Departments</div>
            <RadzenListBox TValue="object"
                           AllowFiltering="false"
                           FilterCaseSensitivity="FilterCaseSensitivity.CaseInsensitive"
                           Change=@(args => OnChange(args, "DepartmentList"))
                           Multiple="true"
                           Placeholder="Select All"
                           Data=@Departments
                           TextProperty="DepartmentName" ValueProperty="DepartmentID"
                           Style="max-height:100px;overflow:scroll;width:160px;font-size:12pt" />
        </div>
    </div>
</EditForm>

<h4>Selected Facilities</h4>
@string.Join(";", (IEnumerable<int>)SelectedFacilites)
<h4>Selected Departments</h4>
@string.Join(";", (IEnumerable<string>)SelectedDepartments)

Code Behind

namespace VibeDashboardsCore.Components
{
    public partial class Counter
    {

        public string formname = "formname";

        public List<DepartmentItem> Departments = new List<DepartmentItem> { new DepartmentItem { DepartmentID = "30", DepartmentName = "Parts" }
                                                                              , new DepartmentItem { DepartmentID = "40", DepartmentName = "Service" }
                                                                            };

        public List<FacilityItem> Facilities = new List<FacilityItem> { new FacilityItem { FacilityID = 1, FacilityName = "fac1" },
                                                                     new FacilityItem { FacilityID = 2, FacilityName = "fac2" },
                                                                     new FacilityItem { FacilityID = 3, FacilityName = "fac3" },
                                                                     new FacilityItem { FacilityID = 4, FacilityName = "fac4" },
                                                                     new FacilityItem { FacilityID = 5, FacilityName = "fac5" },
                                                                     new FacilityItem { FacilityID = 6, FacilityName = "fac6" },                                                                  
                                                                     new FacilityItem { FacilityID = 120, FacilityName = "fac7" }
                                                                   };


        //would change from using to using @bind-Values if i could get it to work
        private IEnumerable<int> SelectedFacilites = Enumerable.Empty<int>();
        private IEnumerable<string> SelectedDepartments = Enumerable.Empty<string>();

        //I assume this is how we are supposed to handle multiple listboxes on the same page.. including other dropdowns
        //Ye old switchboard approach.
        public  void OnChange(object value, string name)
        {
          
            if (name == "FacilityList")
            { 
                SelectedFacilites = (IEnumerable<int>)value;
            }
            if (name == "DepartmentList")
            {
                SelectedDepartments = (IEnumerable<string>)value;
            }
        }   
    }
}
2

Appears I was overthinking this works without the OnChange Event.

Added to code behind

  protected override Task OnParametersSetAsync()
        {
            SelectedFacilites = Facilities.Select(f => f.FacilityID).ToList();
            SelectedDepartments = Departments.Select(d => d.DepartmentID).ToList();

            return Task.FromResult(true);
        }

view

<EditForm Model="@formname">
    <div class="row">
        <div class="col-md-2 ">
            <div>Select Facilities</div>
            <RadzenListBox  TValue="IEnumerable<int>"
                           AllowFiltering="false"
                           FilterCaseSensitivity="FilterCaseSensitivity.CaseInsensitive"
                           @bind-Value="@SelectedFacilites"
                           Multiple="true"
                           Data=@Facilities
                           TextProperty="FacilityName" ValueProperty="FacilityID"
                           Placeholder="Select All"
                           
                           Style="max-height:100px;overflow:scroll;width:160px" />
        </div>
        <div class="col-md-2">
            <div>Select Departments</div>
            <RadzenListBox TValue="IEnumerable<string>"
                           AllowFiltering="true"
                           FilterCaseSensitivity="FilterCaseSensitivity.CaseInsensitive"
                           @bind-Value="@SelectedDepartments"
                           Multiple="true"
                           Placeholder="Select All"
                           Data=@Departments
                           TextProperty="DepartmentName" ValueProperty="DepartmentID"
                           Style="max-height:100px;overflow:scroll;width:160px;font-size:12pt" />
        </div>
    </div>
</EditForm>

I would still like to know how this example works with both @bind-Value and OnChange


from sample 
 <div class="col-xl-6 mb-5">
                <h3>ListBox with multiple selection</h3>
                <RadzenListBox AllowFiltering="true" FilterCaseSensitivity="FilterCaseSensitivity.CaseInsensitive" @bind-Value=@multipleValues Multiple="true" Data=@customers 
                    TextProperty="CompanyName" ValueProperty="CustomerID" Change=@(args => OnChange(args, "ListBox with multiple selection")) Style="height:200px" />
            </div>

when i try this i get a null reference exception that seems to indicate it can't find the OnChange function even though it is in the code behind and vs can find it for hover over ETC