CreditCardSpecification.cs

public class CreditCardSpecification : Specification<PurchaseTestInput>
{
    private readonly PurchaseTestInput purchaseTestInput;

    public CreditCardSpecification(PurchaseTestInput purchaseTestInput)
    {
        this.purchaseTestInput = purchaseTestInput;
    }  

    public override bool IsSatisfiedBy(PurchaseTestInput entity)
    {
        return !string.IsNullOrEmpty(this.purchaseTestInput.CreditCardNumber);
    }
}