>Assume this class:

public partial class Library : EntityObject
>    {
>         public Library Parent
>        {
>            get { return new Library();}
>         }
>    }

As you may guess given the EntityObject inheritance the really thing is more complex, generated by the EDMX builder and cannot be modified. I'd like it to implement an Interface:

public interface ITreeview
>    {
>        ITreeview Parent { get; }
>    }

But (I suppose expectedly) adding this won't compile:

public partial class Library : ITreeview
>    {
>    }

Is there anyway of achieving what I want ?

And yes, that should compile, as long as you implement ITreeview in your custom partial class.