aggregate.cs

public abstract class AggregateRoot
{
    private IList<Guid> Operations { get; } = new List<Guid>();

    public void SetOperation(Guid operationId)
    {
        if (Operations.Contains(operationId)) throw new InvalidOperationException("Duplicate Operation.");
        Operations.Add(operationId);
    }
}

public class Customer : AggregateRoot
{
    private Guid _customerId;
    private string _name;
    private Region _region;
    private bool _active;
    private PricingLevel _pricingLevel;

    public void ChangePricingLevel(PricingLevel pricingLevel)
    {
        _pricingLevel = pricingLevel;
    }
}