public class MainPageViewModel
{
    public ReactiveProperty<string> Input { get; }
    public ReadOnlyReactiveProperty<string> Output { get; }
    public MainPageViewModel()
    {
        Input = new ReactiveProperty<string>("");
        Output = Input
            .Delay(TimeSpan.FromSeconds(1))
            .Select(x => x.ToUpper())
            .ToReadOnlyReactiveProperty();
    }
}

public class AViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private string _name;
    public string Name
    {
        get => _name;
        set
        {
            _name = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Name)));

            // Update a command status
            DoSomethingCommand.RaiseCanExecuteChanged();
        }
    }

    private string _memo;
    public string Memo
    {
        get => _memo;
        set
        {
            _memo = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Memo)));

            // Update a command status
            DoSomethingCommand.RaiseCanExecuteChanged();
        }
    }

    // DelegateCommand is plane ICommand implementation.
    public DelegateCommand DoSomethingCommand { get; }

    public AViewModel()
    {
        DoSomethingCommand = new DelegateCommand(
            () => { ... },
            () => !string.IsNullOrEmpty(Name) && !string.IsNullOrEmpty(Memo)
        );
    }
}

public class AViewModel
{
    public ReactiveProperty<string> Name { get; }
    public ReactiveProperty<string> Memo { get; }
    public ReactiveCommand DoSomethingCommand { get; }

    public AViewModel()
    {
        Name = new ReactiveProperty<string>()
            .SetValidateNotifyError(x => string.IsNullOrEmpty(x) ? "Invalid value" : null);
        Memo = new ReactiveProperty<string>()
            .SetValidateNotifyError(x => string.IsNullOrEmpty(x) ? "Invalid value" : null);
        DoSomethingCommand = new[]
            {
                Name.ObserveHasErrors,
                Memo.ObserveHasErrors,
            }
            .CombineLatestValuesAreAllFalse()
            .ToReactiveCommand()
            .WithSubscribe(() => { ... });
    }
}