Entity Framework: ObjectSet and its (generics) variance

Question

I use: EntityFramework + POCO

Here is the thing:

public interface IBaseType
{
    int Id { get; set; }
}

public class BaseType : IBaseType
{
    public virtual int Id { get; set; }
}

public class DerivedType : BaseType
{
}

The problem:

public class EntityFetcher<T> where T : BaseType
{
    public object GetById(int id)
    {
        ObjectSet<T> objectSet = (ObjectSet<T>)GetTheObjectSet(typeof(T)); 

        return objectSet.SingleOrDefault((o) => o.Id == id);
    }
}

If T is BaseType this all works perfectly, but: The problem is that in EntityFramework when one class inherits another they share the ObjectSet and, therefore, if T is DerivedType then the GetTheObjectSet returns ObjectSet<BaseType>, which cannot be cast to ObjectSet<DerivedType>.

Is there a way to actually cast this this thing or somehow else execute the SingleOrDefault? Can those things be cast using the IObjectSet<> and IBaseType?

Answer 1

I checked one of my test projects which is currently far away from buildable state but this worked for me before:

public interface IEntity
{
    int Id { get; }
    byte[] Timestamp { get; set; }
}

public abstract class Entity : IEntity
{
    public int Id { get; set; }
    public byte[] Timestamp { get; set; }
}

public class PocoData : Entity
{
   ...
}

public class Repository<T> : IRepository<T> where T : class, IEntity 
{
    protected ObjectContext Context;
    protected ObjectSet<T> ObjectSet;

    public Repository(ObjectContext context)
    {
        Context = context;
        ObjectSet = context.CreateObjectSet<T>();
    }

    public virtual T GetById(int id)
    {
        return ObjectSet.SingleOrDefault(o => o.Id == id);
    }

    ...
}

The main point is that Entity class is not modeled in EDMX file. All entities modeled in EDMX file has its own Id and Timestamp but my POCO classes use shared base class. I used Repository<PocoData> without any problem.

Answer 2

I think you're looking for this:

public T GetById(int id)
{
    ObjectSet<T> objectSet = (ObjectSet<T>)GetTheObjectSet(typeof(T)); 

    return objectSet.OfType<T>().SingleOrDefault((o) => o.Id == id);
}

The OfType method of an ObjectQuery (which ObjectSet derives from) will return objects of the derived type.

Answer 3

The answer to this casting problem was as follows:

public T GetById(int id)
{
    // The line that throws InvalidCast with T = DerivedType
    //ObjectSet<T> objectSet = (ObjectSet<T>)GetTheObjectSet(typeof(T));

    // This is a valid cast since ObjectSet<T> is derived from ObjectQuery
    ObjectQuery objectQuery = (ObjectQuery)GetTheObjectSet(typeof(T));
    return objectQuery.OfType<T>().SingleOrDefault((e) => e.Id == id);
}

The solution was to cast ObjectSet to ObjectQuery and do the query there. The most important part here is that ObjectQuery is not generic, so the cast goes through fine.

I must give some credits to Bennor McCarthy as he was the one to point me to OfType + casting to ObjectQuery (the fact that ObjectSet : ObjectQuery). Thanks!