gistfile1.cs

class ClientA
{
    public virtual List<Invoice> Invoices { get; set; }

    //...

    public Product GetMostBuyedProduct(DateTime from, DateTime to) 
    {
        //...
    }
}


class ClientB
{
    public InvoiceRepository InvoiceRepository { get; set; }

    //...

    public Product GetMostBuyedProduct(DateTime from, DateTime to) 
    {
        InvoiceRepository.GetMostBuyedProduct(this.Id, from, to);
    }
}