validation.cs

public class Handler : IAsyncRequestHandler<Envelope<Command>, Unit>
{
    private readonly CustomeRepository _repository;

    public Handler(CustomeRepository repository)
    {
        _repository = repository;
    }

    public async Task<Unit> Handle(Envelope<Command> message)
    {
        if (message.Body.CustomerId == Guid.Empty) throw new InvalidOperationException("CustomerId cannot be empty.");

        var customer = _repository.Get(message.Body.CustomerId);
        try
        {
            customer.SetOperation(message.MessageId);
        }
        catch (InvalidOperationException)
        {
            return Unit.Value;
        }

        customer.ChangePricingLevel(message.Body.PricingLevel);
        await _repository.SaveAsync(customer);

        return Unit.Value;
    }
}