✅ conditional input between two controls avalonia

<ComboBox
    Name="SelectUser"
    Classes="SelectUserCombo"
    Grid.Row="1"
    Grid.Column="0"
    SelectedItem="{Binding SelectedUsername}">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Name}" />
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

public partial class SelectUserViewModel : ViewModelBase
{
    public List<string> _usernames;
    public string? _selectedUsername;
    public string? _newUser;
    public IObservableList<User?>? _currentUsers;

    public SelectUserViewModel()
    {
        AvaloniaXamlLoader.Load(this);

        CurrentUsers = UserQuery.GetUsernames();
        ComboBox comboBox = this.
        comboBox.ItemsSource = CurrentUsers;

        IObservable<bool> selectedUsernameOk = this.WhenAnyValue(
            x => x.SelectedUsername,
            x => !string.IsNullOrEmpty(x));

        IObservable<bool> okEnabled = selectedUsernameOk;

        Ok = ReactiveCommand.Create(
            ReturnSelectedUser,
            okEnabled);
        Cancel = ReactiveCommand.Create(() => { 
            Environment.Exit(0); 
        });
    }

public partial class SelectUserView : UserControl
{
    private List<User?>? Usernames;

    public SelectUserView()
    {
        InitializeComponent();
    }

    private void InitializeComponent()
    {
        AvaloniaXamlLoader.Load(this);
        Usernames = UserQuery.GetUsernames();
        ComboBox comboBox = this.Find<ComboBox>("SelectUser");
        comboBox.ItemsSource = Usernames;
    }
}