<ListBox
  Margin="5"
  ItemsSource="{Binding ListItems}"
  SelectedItem="{Binding SelectedItem, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
  <i:Interaction.Triggers>
    <i:EventTrigger EventName="SelectionChanged">
      <command:EventToCommand
        Command="{Binding ItemsSelectionChangedCommand}"
        CommandParameter="{Binding SelectedItem}"/>
    </i:EventTrigger>
  </i:Interaction.Triggers>
</ListBox>

public partial class MainWindow
{
  public MainWindow()
  {
    InitializeComponent();
    DataContext = new AuxiliaryClass();
  }
}

public class AuxiliaryClass : INotifyPropertyChanged
{
  public AuxiliaryClass()
  {
    ListItems = typeof(Colors).GetProperties(BindingFlags.Public | BindingFlags.Static)
      .Select(f => f.Name)
      .ToList();
    ItemsSelectionChangedCommand = new RelayCommand(
      ItemsSelectionChanged,
      o => o is string s && s.StartsWith("D", StringComparison.OrdinalIgnoreCase)
    );
    SelectedItem = ListItems.First();
  }

  public IList<string> ListItems { get; }
  public event PropertyChangedEventHandler PropertyChanged;

  string _selectedItem;
  public string SelectedItem
  {
    get => _selectedItem;
    set
    {
      _selectedItem = value;
      OnPropertyChanged();
      Console.WriteLine("SelectedItem changed to {0}", SelectedItem);
    }
  }

  [NotifyPropertyChangedInvocator]
  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }
  public RelayCommand ItemsSelectionChangedCommand { get; }

  void ItemsSelectionChanged(object parameter)
  {
    Console.WriteLine("Selection changed: {0}", parameter ?? "null");
  }
}

 <Button  Content="Button" Command="{Binding Add}"

public partial class MainWindow
{
  public MainWindow()
  {
    InitializeComponent();
    DataContext = new TestModel(o=>MessageBox.Show((string)o));
  }
}

public class TestModel : INotifyPropertyChanged
{
  public TestModel(Action<object> showText)
  {
    ShowTextCommand = new MyCommand(showText, o=>!string.IsNullOrEmpty(Text));
  }

  public event PropertyChangedEventHandler PropertyChanged;

  string _text;
  public string Text
  {
    get => _text;
    set
    {
      _text = value;
      OnPropertyChanged();
      ShowTextCommand.RaiseCanExecuteChanged();
    }
  }

  protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
  {
    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
  }

  public MyCommand ShowTextCommand { get; }
}

public class MyCommand : ICommand
{
  readonly Action<object> _onExecute;
  readonly Func<object, bool> _canExecute;

  public MyCommand(Action<object> onExecute, Func<object, bool> canExecute)
  {
    _onExecute = onExecute ?? throw new ArgumentNullException(nameof(onExecute));
    _canExecute = canExecute;
  }

  public bool CanExecute(object parameter)
  {
    return _canExecute==null || _canExecute(parameter);
  }

  public void Execute(object parameter)
  {
    _onExecute(parameter);
  }

  public event EventHandler CanExecuteChanged;
  
  public void RaiseCanExecuteChanged()
  {
    CanExecuteChanged?.Invoke(this, EventArgs.Empty);
  }
}

<Grid>
  <Grid.RowDefinitions>
    <RowDefinition />
    <RowDefinition />
  </Grid.RowDefinitions>
  <TextBox
    VerticalAlignment="Center"
    Text="{Binding Text, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
  <Button
    Grid.Row="1"
    Content="Show Text"
    VerticalAlignment="Center"
    HorizontalAlignment="Center"
    Padding="3"
    Command="{Binding ShowTextCommand}"
    CommandParameter="{Binding Text}"/>
</Grid>