handler.cs

public class ChangeCustomerPricingLevel : IAsyncRequest
{
    public Guid CustomerId { get; }
    public PricingLevel PricingLevel { get; }

    public ChangeCustomerPricingLevel(Guid customerId, PricingLevel pricingLevel)
    {
        CustomerId = customerId;
        PricingLevel = pricingLevel;
    }
}

public class CustomerPricingLevelChanged : IAsyncNotification
{
    public Guid CustomerId { get; }
    public PricingLevel PricingLevel { get; }

    public CustomerPricingLevelChanged(Guid customerId, PricingLevel pricingLevel)
    {
        CustomerId = customerId;
        PricingLevel = pricingLevel;
    }
}

public class ChangeCustomerPricingLevelHandler : IAsyncRequestHandler<Envelope<ChangeCustomerPricingLevel>, Unit>
{
    private readonly IMediator _mediator;
    private readonly CustomerContext _db;

    public ChangeCustomerPricingLevelHandler(IMediator mediator, CustomerContext db)
    {
        _mediator = mediator;
        _db = db;
    }

    public async Task<Unit> Handle(Envelope<ChangeCustomerPricingLevel> message)
    {
        using (var transaction = _db.Database.BeginTransaction())
        {
            _db.IdempotentIds.Add(new Idempotent { IdempotentId = message.MessageId });

            try
            {
                await _db.SaveChangesAsync();
            }
            catch (MySqlException ex)
            {
                // Duplicate Key Error Number
                if (ex.Number == 1062) return Unit.Value;
                throw;
            }

            var customer = _db.Customers.SingleOrDefault(x => x.CustomerId == message.Body.CustomerId);
            if (customer == null) throw new InvalidOperationException();

            customer.PrivingLevel = message.Body.PricingLevel;
            await _db.SaveChangesAsync();
            transaction.Commit();
        }

        var evnt = new CustomerPricingLevelChanged(customer.CustomerId, message.Body.PricingLevel);
        var envelop = new Envelope<CustomerPricingLevelChanged>(message.CorrelationId, evnt);
        await _mediator.PublishAsync(envelop);

        return Unit.Value;
    }
}