moostafaa

public class OrderWithoutInvoiceSpecification : Specification<SaleDocument>
{
    ...
}
public class ApprovedOrderWithoutInvoiceSpecification : AndSpecification<SaleDocument>
{
    public ApprovedOrderWithoutInvoiceSpecification()
        : base(new ApprovedOrderSpecification(), new OrderWithoutInvoiceSpecification())
    {}
}

moostafaa

public decimal GetDiscerpancyAmount(ISpecification<SaleDocument> spec) =>
  spec.IsSatisfiedBy(this) ? ConfirmedPrice - DocumentNetPrice : 0;

moostafaa

public class SaleDocument
{
....
  public decimal GetDiscerpancyAmount(ISpecification<SaleDocument> spec) =>
    spec.IsSatisfiedBy(this) ? ConfirmedPrice - DocumentNetPrice : 0;
....
}

public class SaleDocumentService
{