Generic return types from abstract/virtual methods.

Question

I have a relationship between two base classes:

public abstract class RecruiterBase<T>
{
  // Properties declare here
  // Constructors declared here

  public abstract IQueryable<T> GetCandidates();
}

public abstract class CandidateBase<T>
{
  // Properties declare here
  // Constructors declared here
}

And their concrete implementations as such:

public class CandidateA : CandidateBase<CandidateA>
{
  // Constructors declared here
}

public class RecruiterA : RecruiterBase<RecruiterA>
{
  // Constructors declared here

  // ----HERE IS WHERE I AM BREAKING DOWN----
  public override IQueryable<CandidateA> GetCandidates()
  {
     return from c in db.Candidates
            where c.RecruiterId == this.RecruiterId
            select new CandidateA
            {
              CandidateId = c.CandidateId,
              CandidateName = c.CandidateName,
              RecruiterId = c.RecruiterId
            };
  }
}

Per MSDN documentation http://msdn.microsoft.com/en-us/library/ms379564%28VS.80%29.aspx (about half way down) and a similiar (but not identical) questoin on SO http://stackoverflow.com/questions/675857/c-specifying-the-return-type-of-an-abstract-method-from-a-base-class-according

I can make use of my concreate implementation for the return type of my overridden method GetCandidates but that is not what I want, I want to make use of the concrete implementation of a different abstract class. This is a parent/child database relationship. Is what I am trying to achieve possible? I currently get a compile time error that my GetCandidates return type does not match.

Thanks

Answer 1

It looks like you need to define multiple generic types, with one possibly being constrained to derive from CandidateBase.

Try something like this:

public abstract class RecruiterBase<T, C> where C : CandidateBase
{
  // Properties declare here
  // Constructors declared here

  public abstract IQueryable<C> GetCandidates();
}

public abstract class CandidateBase<T>
{
  // Properties declare here
  // Constructors declared here
}

public class CandidateA : CandidateBase<CandidateA>
{
  // Constructors declared here
}

public class RecruiterA : RecruiterBase<RecruiterA, CandidateA>
{
  public override IQueryable<CandidateA> GetCandidates()
  {
  return from c in db.Candidates
   where c.RecruiterId == this.RecruiterId
   select new CandidateA
   {
     CandidateId = c.CandidateId,
     CandidateName = c.CandidateName,
     RecruiterId = c.RecruiterId
   };
  }
}

Edit Included Chris's correction