CounterStore.cs

class CounterStore : INotifyPropertyChanged
{
  private readonly CounterReducer reducer = new CounterReducer();
  private CounterState state;

  public CounterStore(CounterState initialState)
  {
    state = initialState;
  }

  public CounterState State
  {
    get { return state; }
    private set { state = value; PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(State))); }
  }

  public void Dispatch(object action)
  {
    State = reducer.Reduce(State, action);
  }

  public event PropertyChangedEventHandler PropertyChanged;
}