CustomerRegUseCase.cs

public class CustomerRegistrationUseCase
{
	public async Task RegisterCustomer(
		CustomerRegistrationRequest customerRegistrationRequest)
	{
		var customer = await registeredCustomers.GetByEmail(
			customerRegistrationRequest.Email);

		if (customer == null)
		{
			var newCustomer = customerRegistrationRequest.ToNewCustomer();
			await registeredCustomers.Add(newCustomer);	
			
			await eventBus.Publish(
				new NewCustomerRegistered(newCustomer)));
		}
		else
		{
			throw new CustomerAlreadyExists(customer);
		}		
	}
}