public abstract class BaseEntity
     {
         public virtual int ID { get; private set; }
     }
     public class ORDER : BaseEntity
     {
         public virtual int OrderNumber { get; set; }
         public virtual string Comment { get; set; }
         public virtual DateTime DateOfPlace { get; set; }
         public virtual int NumberOfItems { get; set; }

         public virtual CUSTOMER Customer { get; set; }
         public virtual PRODUCT Product { get; set; }
     }

    public class PRODUCT : BaseEntity
    {
        public virtual string Code { get; set; }
        public virtual int ProductNumber { get; set; }
        public virtual string Name { get; set; }
        public virtual double Price { get; set; }
    }

orderNumberColumn.DataPropertyName="OrderNumber";
            productNameColumn.DataPropertyName="Product.Name";
            countColumn.DataPropertyName="NumberOfItems";
            orderDateColumn.DataPropertyName="DateOfPlace";
            commentColumn.DataPropertyName="Comment";