public sealed class DelegateCommand<T> : Command
{
    private readonly Action<T> execute;
    private readonly Func<T, bool> canExecute;

    public DelegateCommand(Action<T> execute, Func<T, bool> canExecute)
    {
        this.execute = execute;
        this.canExecute = canExecute;
    }

    public override bool CanExecute(object parameter)
    {
        return (canExecute != null) ? canExecute((T)parameter) : true;
    }

    public override void Execute(object parameter)
    {
        execute((T)parameter);
    }
}


public sealed class MainWindowViewModel : INotifyPropertyChanged {
    private string city;

    public MainWindowViewModel(... service) {
        this.service = service;

        ListCommand = new DelegateCommand<object>(obj => ListRealties, null);
        Realties = new ObservableCollection<Realty>();
    }

    public event PropertyChangedEventHandler PropertyChanged;

    public string City {
        get { return city; }
        set {
            if (value != city) {
                city = value;
                var handler = PropertyChanged;
                if (handler != null)
                    handler(this, new PropertyChangedEventArgs("City"));
            }
        }
    }

    public ObservableCollection<Realty> Realties { get; private set; }

    private void ListRealties() {
        Realties.Clear();
        foreach (var realty in service.GetAllProperties(City))
            Realties.Add(new Realty(realty.Condition, realty.Street));
    }
}



<Window>
    <TextBox Text="{Binding City}"/>
    <ListBox ItemsSource="{Binding Realties}"/>
    <Button Command="{Binding ListCommand}" Content="Auflisten"/>
</Window>