NUnit Test Domain events

Question

I used this for implementation domain events https://learn.microsoft.com/en-us/dotnet/architecture/microservices/microservice-ddd-cqrs-patterns/domain-events-design-implementation

I have event, handle and transaction infrastructure for dispatch event. But how can I write tests for handler? How can I call handler from unit test without "savecontenxt() " which used in API for this.

for example event and handle

event

public class OrderStartedDomainEvent : INotification
{
    public string UserId { get; }
    public int CardTypeId { get; }
    public string CardNumber { get; }
    public string CardSecurityNumber { get; }
    public string CardHolderName { get; }
    public DateTime CardExpiration { get; }
    public Order Order { get; }

    public OrderStartedDomainEvent(Order order,
                                   int cardTypeId, string cardNumber,
                                   string cardSecurityNumber, string cardHolderName,
                                   DateTime cardExpiration)
    {
        Order = order;
        CardTypeId = cardTypeId;
        CardNumber = cardNumber;
        CardSecurityNumber = cardSecurityNumber;
        CardHolderName = cardHolderName;
        CardExpiration = cardExpiration;
    }
}

eventin entity

var orderStartedDomainEvent = new OrderStartedDomainEvent(this, //Order object
                                                          cardTypeId, cardNumber,
                                                          cardSecurityNumber,
                                                          cardHolderName,
                                                          cardExpiration);
this.AddDomainEvent(orderStartedDomainEvent);

handler

                   : INotificationHandler<OrderStartedDomainEvent>
{
    private readonly ILoggerFactory _logger;
    private readonly IBuyerRepository<Buyer> _buyerRepository;
    private readonly IIdentityService _identityService;

    public ValidateOrAddBuyerAggregateWhenOrderStartedDomainEventHandler(
        ILoggerFactory logger,
        IBuyerRepository<Buyer> buyerRepository,
        IIdentityService identityService)
    {
        // ...Parameter validations...
    }

    public async Task Handle(OrderStartedDomainEvent orderStartedEvent)
    {
        var cardTypeId = (orderStartedEvent.CardTypeId != 0) ? orderStartedEvent.CardTypeId : 1;
        var userGuid = _identityService.GetUserIdentity();
        var buyer = await _buyerRepository.FindAsync(userGuid);
        bool buyerOriginallyExisted = (buyer == null) ? false : true;

        if (!buyerOriginallyExisted)
        {
            buyer = new Buyer(userGuid);
        }

        buyer.VerifyOrAddPaymentMethod(cardTypeId,
                                       $"Payment Method on {DateTime.UtcNow}",
                                       orderStartedEvent.CardNumber,
                                       orderStartedEvent.CardSecurityNumber,
                                       orderStartedEvent.CardHolderName,
                                       orderStartedEvent.CardExpiration,
                                       orderStartedEvent.Order.Id);

        var buyerUpdated = buyerOriginallyExisted ? _buyerRepository.Update(buyer)
                                                                      : _buyerRepository.Add(buyer);

        await _buyerRepository.UnitOfWork
                .SaveEntitiesAsync();

        // Logging code using buyerUpdated info, etc.
    }
}```