CounterReducer.cs

class CounterReducer
{
  public CounterState Reduce(CounterState state, object action)
  {
    switch (action)
    {
      case IncrementAction i:
        return ProcessIncrement(state);
      case DecrementAction d:
        return ProcessDecrement(state);
      default:
        throw new ArgumentException("action is not supported");
    }
  }

  private CounterState ProcessIncrement(CounterState state)
  {
    return new CounterState(state.Count + 1);
  }

  private CounterState ProcessDecrement(CounterState state)
  {
    return new CounterState(state.Count - 1);
  }
}