CounterReducer.cs

// Defined separately from the grain, this reducer can be taken out and shared with
// a Xamarin / XAML app using Redux.NET as the clientside store
public static CounterState Reducer(CounterState state, IAction action)
{
    switch (action)
    {
        case IncrementCounterAction a:
            return new CounterState(state) { Count = (state?.Count ?? 0) + 1 };

        case DecrementCounterAction a:
            return new CounterState(state) { Count = (state?.Count ?? 0) - 1 };

        case StartCounterAction a:
            return new CounterState(state) { Started = true };

        case StopCounterAction a:
            return new CounterState(state) { Started = false };

        default:
            return state;
    }
}