Mock Entity Framework with in Memory Test Double

Question

So, I pretty much feel somewhat frustrated on Microsoft suggested tutorials - I believe there are contracts that being missed but i am surprised that nowhere clue to be found.

I am trying to implement in memory test double for a certain Entity Framework 5 implementation. I tried MSDN's tutorial on EF 6 and it works pretty much (and obviously data only persistent for a single context - cause DbSet only being referenced in given context). However, the tutorial that suggested by Microsoft for EF 5 and earlier , it simply doesn't cut to it. I bet missing some contracts cause I get "Cannot implicitly convert type ..FakeSomeModelDbSet to System.Data.Entity.DbSet" ! But don't know what should I put on the : (extend) part and what else i should override.

Basically, here is my base mock DbSet implementation:

abstract class MockDbSet<T> : IDbSet<T> where T : class
{
    #region Properties

    protected ObservableCollection<T> _data;
    protected IQueryable _query;

    Type IQueryable.ElementType
    {
        get { return this._query.ElementType; }
    }

    System.Linq.Expressions.Expression IQueryable.Expression
    {
        get { return this._query.Expression; }
    }

    IQueryProvider IQueryable.Provider
    {
        get { return this._query.Provider; }
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return _data.GetEnumerator();
    }

    IEnumerator<T> IEnumerable<T>.GetEnumerator()
    {
        return _data.GetEnumerator();
    }

    #endregion

    #region Public Methods

    public MockDbSet()
    {
        this._data = new ObservableCollection<T>();
        this._query = _data.AsQueryable<T>();
    }

    public T Find(params object[] keyValues)
    {
        return this._data.Single(x => this._GetObjectKey(x) == (Guid)keyValues.Single());
    }

    public T Add(T entity)
    {
        this._data.Add(entity);
        return entity;
    }

    public T Remove(T entity)
    {
        this._data.Remove(entity);
        return entity;
    }

    public T Attach(T entity)
    {
        this._data.Add(entity);
        return entity;
    }

    public T Detach(T entity)
    {
        this._data.Remove(entity);
        return entity;
    }

    public T Create()
    {
        return Activator.CreateInstance<T>();
    }

    public TDerivedEntity Create<TDerivedEntity>() where TDerivedEntity : class, T
    {
        return Activator.CreateInstance<TDerivedEntity>();
    }

    public ObservableCollection<T> Local
    {
        get { return this._data; }
    }

    #endregion

    #region Protected Methods

    protected abstract Guid _GetObjectKey(T entity);

    #endregion
}

And here's one of it's implementation that intended to replace the original DbSet :

class FakeSomeModelbSet : MockDbSet<SomeModel>
{
    protected override Guid _GetObjectKey(SomeModel entity)
    {
        return entity.SomeModelId;
    }
}

And here how I'm going to hook the entire things in a certain DbContext ..

class MockingContext:OriginContext
{
    public MockCpdContext()
    {
        base.SomeModel = new FakeSomeModelbSet ();
    }
}

Don't ask me to use Moq adding a library is a no go, as I'm not the person in charge just a lowly coding drone. If you ask me the implementation of the OriginContext, it was generated by Database-first approach.. so.. i can, but that won't be much of help.

To be honest, the clue lies on the difference on tutorial code ( public class FakeDbSet<T> : IDbSet<T> ) and the original DbSet implementation ( public class DbSet<TEntity> : DbQuery<TEntity>, IDbSet<TEntity>, IQueryable<TEntity>, IEnumerable<TEntity>, IQueryable, IEnumerable, IInternalSetAdapter where TEntity : class ) signatures.. but dunno what to do.. really.

Answer 1

Well, i just realized that in Entity Framework 5 DbSet only have a single constructor with internal modifier . It is not possible to inherit DbSet itself directly, it is only possible through replacing the DbSet in DbContext with IDbSet that is only possible to edit by modifying the .Context.tt file.