Cocktail/Temphire - Unit of Work & Repository Architecture

I am just studying the Temphire reference application and came across a topic I do not fully understand.
Maybe you can bring some light on the problem and recommend a best practice approach.

Let's say I would like to extend the StaffingResourceRepository by some custom data selection method, e.g.

public async Task<IEnumerable<StaffingResource>> FindAll()
{
// Get all StaffingResources
var query = EntityManager.StaffingResources;
return await query.ExecuteAsync();
}

In the StaffingResourceDetailViewModel I would like to use this method but I can't because
in the ResourceMgtUnitOfWork the StaffingResources is defined as IRepository<StaffingResource> and not as StaffingResourceRepository.

Could you please explain why you use IRepository<StaffingResource> and not StaffingResourceRepository in the ResourceMgtUnitOfWork
and what is the best approach if I would like to use the "custom" StaffingResourceRepository in a ViewModel instead of the generic one.

Thank you for your assistance
Daniel

Thank you for your reply.
The concept of interfaces is known but if working with demo applications it’s not always easy to understand if something was implemented just because for the sake of convenience.
However. If I understood you right. You recommend something like this, right?

Thanks
Daniel

namespace DomainServices.Repositories
{
    public interface IStaffingResourceRepository : IRepository<StaffingResource>
    {
        Task<IEnumerable<StaffingResource>> FindAll();
    }
}

namespace DomainServices.Repositories
{
    public class StaffingResourceRepository : Repository<StaffingResource>, IStaffingResourceRepository
    {
        public StaffingResourceRepository(IEntityManagerProvider<TempHireEntities> entityManagerProvider)
            : base(entityManagerProvider)
        {
        }

        public new TempHireEntities EntityManager
        {
            get { return (TempHireEntities) base.EntityManager; }
        }

        public Task<IEnumerable<StaffingResource>> FindAll()
        {
            // Get all StaffingResources
            var query = EntityManager.StaffingResources;
            return query.ExecuteAsync();
        }

        protected override IEntityQuery GetKeyQuery(params object[] keyValues)
        {
            return EntityManager.StaffingResources
               .Where(r => r.Id == (Guid) keyValues[0])
               .Include(r => r.Addresses)
               .Include(r => r.PhoneNumbers);
        }
    }
}

namespace DomainServices
{
    public interface IResourceMgtUnitOfWork : IUnitOfWork
    {
        // Factories
        IFactory<StaffingResource> StaffingResourceFactory { get; }

        // Repositories
        IRepository<AddressType> AddressTypes { get; }
        IRepository<State> States { get; }
        IRepository<PhoneNumberType> PhoneNumberTypes { get; }
        IRepository<RateType> RateTypes { get; }
        IStaffingResourceRepository StaffingResources { get; }

        // Services
        IStaffingResourceSearchService Search { get; }
    }
}