nhibernate - Strategies for DRY when using ViewModels instead of Entities -

a lot of our pages use custom view models cut versions of nhibernate-mapped entities.

when have global set of entities, reusing small pieces of entity-centric business logic straightforward logic can added entities. however, once have view-specific view models, becomes harder.

here contrived example:

// entity public class sale : entity {     public guid id { get; set; }     public decimal tax { get; set; }     public decimal saleprice { get; set; }     public int totalprice     {         { return tax + saleprice; }     }     ... more properties ... }  // view model public class saleview {     public guid id { get; set; }     public decimal tax { get; set; }     public decimal saleprice { get; set; } }  public class controller {     public action viewsale(guid id)     {         return session.query<sale>()             .where(x => x.id == id)             .select(x => new saleview                 {                     id = x.id,                     tax = x.tax,                     saleprice = x.saleprice                 });     } } 

in example, strategies use reuse "totalprice" calculation performed in entity?

you make "totalprice" static method somewhere, , have accept tax , sale price parameters, can lead quite ugly, inflexible code, particularly number of properties , relationship between properties grows. create seperate "totalpricecalculator" class, deal viewmodel need accept interface both entity , view model implement. lead proliferation of duplicated code, have write out each property 3 times. else can done?

you define total price policy, can inject in entity , viewmodel. fact inject total price policy makes more flexible. implement other policies. thing this:

public interface itotalpricepolicy {     decimal totalprice(decimal tax, decimal salesprice); }  public class totalpricepolicy : itotalpricepolicy {     public decimal totalprice(decimal tax, decimal salesprice)     {         return tax + salesprice;     } }  public class sale : entity {     public guid id { get; set; }     public decimal tax { get; set; }     public decimal saleprice { get; set; }      public decimal totalprice(itotalpricepolicy totalpricepolicy)     {         return totalpricepolicy.totalprice(tax, saleprice);     } }  public class saleviewmodel {     private readonly itotalpricepolicy _totalpricepolicy;      public saleviewmodel(itotalpricepolicy totalpricepolicy)     {         _totalpricepolicy = totalpricepolicy;     }      public guid id { get; set; }     public decimal tax { get; set; }     public decimal saleprice     {         { return _totalpricepolicy.totalprice(tax, saleprice); }     } }  public class homecontroller : controller {     private readonly itotalpricepolicy _totalpricepolicy;      public homecontroller(itotalpricepolicy totalpricepolicy)     {         _totalpricepolicy = totalpricepolicy;     }      public action viewsale(guid id)     {         return session.query<sale>()             .where(x => x.id == id)             .select(x => new saleview(_totalpricepolicy)             {                 id = x.id,                 tax = x.tax             });     } }