Question

Within my MVC3 app I am trying to create a general class (named DdlGet below) to call that gets records for a drop down list (DDL). The code below does as intended however I think I’m overbaking the use of the Generic type T – specifically the line indicated below with the ‘//**’

I have the following code in my Controller

private readonly IGeneralReferenceRepository<StatusType> statusTypeRepository;
...
public StatusController() : this(...new StatusTypeRepository()) {}

public StatusController(...IGeneralReferenceRepository<StatusType> statusTypeRepository)
{
    ...
    this.statusTypeRepository = statusTypeRepository;
}
...
public ViewResult Index()
{
    ...
    //**** The line below passes a variable (statusTypeRepository) of the Generic
    //**** type (StatusType) and additionally calls the class (Helper<StatusType>)
    //**** with the Generic
    indexViewModel.StatusTypes = Helper<StatusType>.DdlGet(statusTypeRepository);

Then in my repository (this defines the implementation for getting the records for the DDL from the database [via Entity Framework method]) – note the general reference Generic interface (IGeneralReferenceRepository)

public class StatusTypeRepository : IStatusTypeRepository, IGeneralReferenceRepository<StatusType>
{
    ...
    public IQueryable<StatusType> All
    {
        get { return context.StatusTypes; }
    }

I have an interface (which corresponds to the All method being called above)

public interface IGeneralReferenceRepository<T>
{
    IQueryable<T> All { get; }
}

And a helper class to get the drop down list records and place into the SelectList

public class Helper<T>
{
   public static SelectList DdlGet(IGeneralReferenceRepository<T> generalReferenceRepository)
   {
       return new SelectList(generalReferenceRepository.All, ...);
   }
}

The issue I have is the line indicated in the first code block above – i.e. the call to the eventual implementation that populates the SelectList

indexViewModel.StatusTypes = Helper<StatusType>.DdlGet(statusTypeRepository); 

As explaned above in the comment (prefixed with //**) this passes a Generic statusTypeRepository which defines the type via the line:-

private readonly IGeneralReferenceRepository<StatusType> statusTypeRepository;

However I’m already defining the type in the Helper Generic class (i.e. the Helper class)

The question I have is can I derive one from the other rather than specifying the generic twice in the call. i.e. can I derive the type specified in the statusTypeRepository from the Helper class type or vice versa

Many thanks
Travis

Answer 1

Rather than having the type parameter on your Helper class, you could put it on the methods within like this:

public class Helper
{
    public static SelectList DdlGet<T>(IGeneralReferenceRepository<T> generalReferenceRepository)
    {
        return new SelectList(generalReferenceRepository.All, ...);
    }
}

Then you can just do

indexViewModel.StatusTypes = Helper.DdlGet(statusTypeRepository); 

and the compiler will handle the type inference.