map.cs

public class BalanceProjection
{
    private readonly IEventMap<Balance> _map;
    public Balance Balance { get; } = new Balance(0, DateTime.UtcNow);

    public BalanceProjection(IStreamStore streamStore, StreamId streamId)
    {
        var mapBuilder = new EventMapBuilder<Balance>();
        mapBuilder.Map<Deposited>().As((deposited, balance) =>
        {
            balance.Add(deposited.Amount);
        });
        mapBuilder.Map<Withdrawn>().As((withdrawn, balance) =>
        {
            balance.Subtract(withdrawn.Amount);
        });

        _map = mapBuilder.Build(new ProjectorMap<Balance>()
        {
            Custom = (context, projector) => projector()
        });

        streamStore.SubscribeToStream(streamId, null, StreamMessageReceived);
    }

    private async Task<object> DeserializeJsonEvent(StreamMessage streamMessage, CancellationToken cancellationToken)
    {
        var json = await streamMessage.GetJsonData(cancellationToken);

        switch (streamMessage.Type)
        {
            case "Deposited":
                return JsonConvert.DeserializeObject<Deposited>(json);
            case "Withdrawn":
                return JsonConvert.DeserializeObject<Withdrawn>(json);
            default:
                throw new InvalidOperationException("Unknown event type.");
        }
    }

    private async Task StreamMessageReceived(IStreamSubscription subscription, StreamMessage streamMessage, CancellationToken cancellationToken)
    {
        var @event = await DeserializeJsonEvent(streamMessage, cancellationToken);
        await _map.Handle(@event, Balance);
    }
}