Resuable Dropdown/ComboBox component for (Calibrun) MVVM

DropDownViewModel.cs

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using Caliburn.Micro;
using Scheduler.Data;

namespace MyApp
{
    public class DropDownViewModel : Screen
    {
        private int _selectedIndex;
        private int _selectedValue;
        private IEnumerable<DropDownItem> _dropDownList;
        private readonly ObservableCollection<DropDownItem> _ddItems = new ObservableCollection<DropDownItem>();

        public IEnumerable<DropDownItem> DropDownList
        {
            get => _dropDownList;
            set
            {
                _dropDownList = value;
                loadDropDownList();
            }
        }

        public int Counter { get; set; } = 0;

        public void AddItem(string itemName, int? itemValue=null)
        {
            var _itemValue = itemValue ?? Counter;

            _ddItems.Add(new DropDownItem
                         {
                             DisplayName = itemName,
                             Value = _itemValue,
                             Index = Counter++
                         });

            Items = _ddItems;
        }

        private void loadDropDownList()
        {
            foreach (var dropDownItem in DropDownList)
            {
                _ddItems.Add(new DropDownItem
                             {
                                 DisplayName = dropDownItem.DisplayName,
                                 Value = dropDownItem.Value,
                                 Index = Counter++
                             });
            }

            Items = _ddItems;
        }

        public void UpdateSelectedItem(int selectedItem, bool byIndex = true)
        {
            var dd = byIndex 
                         ? Items.FirstOrDefault(x => x.Index == selectedItem) 
                         : Items.FirstOrDefault(x => x.Value == selectedItem);

            if (dd?.Index != null)
            {
                _selectedIndex = dd.Index;
                _selectedValue = dd.Value;
                SelectedDisplayName = dd.DisplayName;
                
                NotifyOfPropertyChange(() => SelectedIndex);
                NotifyOfPropertyChange(() => SelectedValue);
                NotifyOfPropertyChange(() => SelectedDisplayName);
            }
        }
        
        public int SelectedValue
        {
            get => _selectedValue;
            set => UpdateSelectedItem(value, byIndex : false);
        }

        public int SelectedIndex
        {
            get => _selectedIndex;
            set => UpdateSelectedItem(value, byIndex : true);
        }

        public ObservableCollection<DropDownItem> Items { get; set; }
        public string SelectedDisplayName { get; set; }
        
    }
    
}

TestProjectView.xaml

.....

<Label Grid.Row="1" Grid.Column="0">My Options List</Label>
<ComboBox Grid.Row="1" Grid.Column="1" ItemsSource="{Binding MyDropDown.Items}" 
          SelectedValuePath="Value" DisplayMemberPath="DisplayName"
          SelectedValue="{Binding SelectedValue}"                            
          IsEditable="False" HorizontalAlignment="Stretch" MaxDropDownHeight="600"

/>

.....

<Label Grid.Row="2" Grid.Column="0">My Options List 2</Label>
<ComboBox Grid.Row="2" Grid.Column="1" ItemsSource="{Binding MyDropDown2.Items}" 
          SelectedValuePath="Value" DisplayMemberPath="DisplayName"
          SelectedValue="{Binding SelectedValue}"                            
          IsEditable="False" HorizontalAlignment="Stretch" MaxDropDownHeight="600"

/>
....

TestProjectViewModel.cs

namespace MyApp
{
    public abstract class TestProjectViewModel : Screen
    {
        public TestProjectViewModel()
        {
            MyDropDown = new DropDownViewModel()
                               {
                                   DropDownList = DropDownHelper.GetMyOptionsList(),
                                   SelectedValue = MySelectedItem > 0 ? MySelectedItem : 0
                               };
                               
           MyDropDown2 = new DropDownViewModel();
           MyDropDown2.AddItem("");
           MyDropDown2.AddItem("Item 1");
           MyDropDown2.AddItem("Item 2");
        }

    }
}