handler.cs

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)
    {
        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();

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

        return Unit.Value;
    }
}