CalculatorGrain.cs

public class CalculatorGrain : Grain<CalculatorGrain.GrainState>, ICalculatorGrain
{
    private CalculatorLogic _logic;

    public class GrainState
    {
        public List<int> Values { get; set; }
    }
 
    public override Task OnActivateAsync()
    {
        _logic = new CalculatorLogic(
                state: State,
                grainFactory: GrainFactory,
                writeState: WriteStateAsync
                );
 
        return base.OnActivateAsync();
    }
 
    public Task EnterValue(int value) => _logic.EnterValue(value);
 
    private Task<int> CalculatSum() => _logic.CalculateSum();
}

CalculatorLogic.cs

public class CalculatorLogic
{
    private readonly CalculatorGrain.GrainState _state;
    private readonly IGrainFactory _grainFactory;
    private readonly Func<Task> _writeState;
 
    public CalculatorLogic(
        CalculatorGrain.GrainState state,
        IGrainFactory grainFactory,
        Func<Task> writeState  
        )
    {
        _state = state;
        _grainFactory = grainFactory;
        _writeState = writeState;
    }
 
    public async Task EnterValue(int value)
    {
         _state.Values.Add(value);
         await _writeState();
    }
 
    public async Task<int> CalculateSum()
    {
        var sum=_state.Values.Sum();
        return Task.FromResult(sum);
    }
}

ICalculatorGrain.cs

public interface ICalculatorGrain : IGrainWithStringKey
{
    Task EnterValue(int value);
    Task<int> CalculateSum();
}