public sealed class GiftCardCreated : AggregateCreatedEvent<Guid>
{
    public GiftCardCreated(Guid aggregateId, DateTime timestamp, decimal initialCredit)
        : base(aggregateId, timestamp)
    {
        InitialCredit = initialCredit;
    }

    public decimal InitialCredit { get; }
}

public class GiftCardDebited : AggregateEvent<Guid>
{
    public GiftCardDebited(Guid aggregateId, int aggregateVersion, DateTime timestamp, decimal amount)
        : base(aggregateId, aggregateVersion, timestamp)
    {
        Amount = amount;
    }

    public decimal Amount { get; }
}

public class GiftCard : Aggregate<Guid>
{
    /// <summary>
    /// Constructor for creating an new gift card from scratch
    /// </summary>
    public GiftCard(decimal initialCredit)
        : base(new GiftCardCreated(Guid.NewGuid(), DateTime.UtcNow, initialCredit))
    { }

    /// <summary>
    /// Constructor for rehydrating gift card from historical events
    /// </summary>
    public GiftCard(Guid id, IEnumerable<IAggregateEvent<Guid>> savedEvents)
        : base(id, savedEvents)
    { }

    /// <summary>
    /// Constructor for rehydrating gift card from a snapshot + historical events after the snapshot
    /// </summary>
    public GiftCard(Guid id, IAggregateSnapshot<Guid> snapshot, IEnumerable<IAggregateEvent<Guid>> savedEvents)
        : base(id, snapshot, savedEvents)
    { }

    public decimal Balance { get; private set; }

    public void Debit(decimal amout)
        => ReceiveEvent(new GiftCardDebited(Id, GetNextVersion(), DateTime.UtcNow, amout));

    protected override void ApplyEvent(IAggregateEvent<Guid> @event)
    {
        if (@event is GiftCardCreated created)
        {
            Balance = created.InitialCredit;
        }
        else if (@event is GiftCardDebited debited)
        {
            if (debited.Amount < 0)
            {
                throw new InvalidOperationException("Negative debit amout is not allowed.");
            }

            if (Balance < debited.Amount)
            {
                throw new InvalidOperationException("Not enough credit");
            }

            Balance -= debited.Amount;
        }
    }
}

public interface IGiftCardRepository
{
    Task SaveGiftCardAsync(GiftCard giftCard);
    Task<GiftCard> FindGiftCardAsync(Guid id);
}

public class GiftCardRepository : AggregateRepository<GiftCard, Guid>, 
    IGiftCardRepository
{
    public GiftCardRepository(IEventStore<GiftCard, Guid> eventStore)
        : base(eventStore)
    { }

    public Task SaveGiftCardAsync(GiftCard giftCard) =>
        SaveAggregateAsync(giftCard);

    public Task<GiftCard> FindGiftCardAsync(Guid id) =>
        FindAggregateAsync(id);
}

services
    .AddScoped<IGiftCardRepository, GiftCardRepository>();

services
    .AddEventSourcing(builder =>
    {
        builder.UseTextFileEventStore<GiftCard, Guid>(x =>
            x.Folder = "C:/Temp/GiftcardEvents");
    });

// create a new gift card with initial credit 100
var giftCard = new GiftCard(100);

// persist the gift card
await _repository.SaveGiftCardAsync(giftCard);

// rehydrate the giftcard
giftCard = await _repository.FindGiftCardAsync(giftCard.Id);

// payments
giftCard.Debit(40); // ==> balance: 60
giftCard.Debit(50); // ==> balance: 10
giftCard.Debit(20); // ==> invalid operation exception