using Inheritance with generics

Question

suppose that i have a base class

class Document
{
//some properties
}

and two derived classes:

public class Order:Document
    {
    //some properties   
    }
public class Request:Document
{
    //some properties   
}

now i want make a layered application that have a service layer. also i want have a base service layer interface such as:

public interface IBaseService<T> where T:Document
{
    IList<T> GetAll();
}

that my layer service classes implement it:

public SaleService:IBaseService<Document>;

So i must define IList<Document> GetAll() to implement base service, but i want my SaleService has IList<Order> GetAll() and IList<Request> GetAll() methods, instead IList<Document> GetAll(). how can i do it?

You can't because you set one IList<T> method in IBaseService<T> interface. — MarcinJuraszek
– MarcinJuraszek, Commented Apr 14, 2013 at 15:51
It sounds like you have to have two different GetAll() methods on the same class, with different return types. You can't do that in C#. Please try to clarify your question. — harpo
– harpo, Commented Apr 14, 2013 at 15:52
yeah i have two different GetAll() methods on same class with different return types. — Masoud
– Masoud, Commented Apr 14, 2013 at 15:55
You should simply use public SaleService:IBaseService<Order> ...; , otherwise the <T> parameter is superfluous. — Henk Holterman
– Henk Holterman, Commented Apr 14, 2013 at 16:01

Lee · Accepted Answer · 2013-04-14 18:31:56Z

4

You will need to implement IBaseService<T> explicitly with the different generic parameters:

public class SalesService : IBaseService<Order>, IBaseService<Request>
{
    IList<Order> IBaseService<Order>.GetAll()
    {
        //return orders
    }

    IList<Request> IBaseService<Request>.GetAll()
    {
        //return requests
    }
}

The implemented methods are only visible through the interface, so you'll have to cast to call them e.g.

IBaseService<Request> service = new SalesService();
IList<Request> request = service.GetAll();

Your client code should only depend on the IBaseService<T> interface, so you shouldn't need to do this very often.

edited Apr 14, 2013 at 18:31

answered Apr 14, 2013 at 15:54

Lee

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3 Comments

Masoud Over a year ago

but by implementing explicitly IBaseService<Order> and IBaseService<Request>, modifier of GetAll methods are private and i couldn't use them on my client side code. and if change modifiers of GetAlls to public, i get this error:"The modifier 'public' is not valid for this item"

Lee Over a year ago

@Masoud - Yes, they're only visible through the interface so you'll need to cast - see the update. You can implement one of the IBaseService<T> interfaces implicitly if you want to call GetAll on the SalesService class directly.

Masoud Over a year ago

thanks, i tested it, it worked. also it worked with IBaseService<Request> service = new SalesService(); but another question, how can i declare IList<Document> instead IList<Request> in last line of code?

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