How to specify a generic type's identifier type (for db queries) along with genric type

Question

Hi there,

I was thinking of implementing a per-object Repository (e.g. a Person repository, a State repository etc.) before deciding on a line of business Repository instead BUT I still have a question regarding the passing in of an ID (identifier) type with a generic.

In a nutshell, a Person may have a GUID identifier, a State/County may have a int identifier, etc. With this in mind, how can I pass along a type that specifies the ID, er, type of the generic being passed in?

Here's my attempt at the interface:

public interface IRepository<T, TID> where T : class where TID : Type
{        
    T Get(TID id);
    IList<T> GetAll();
    void Update(T entity);
    void Insert(T entity);
    void Delete(TID id);
}

You can see that T is the class (Person, State, etc.) and my intent is to have TID be the type that is used for the identifier, i.e. look at T Get(TID id).

This craps out with the compilation error: "The type 'string' cannot be used as type parameter 'TID' in the generic type or method 'DumpWebApp.IRepository'. There is no implicit reference conversion from 'string' to 'System.Type'."

Another way of doing it would be to pass the actual element into the Get and Delete methods, I guess, but that seems a bit clunky.

Thanks for any suggestions - and incidentally, is returning IList acceptable for GetAll()? Why not IQueryable, or T[] ?

Mike K.

Answer 1

Just drop the where TID : Type constraint. That should do the trick.

Answer 2

IQueryable no. IList yes. Array yes depending. IList is safe.

Your TID constraint is saying that TID will be of type Type.

drop that.