>>>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..
>
>The Library class (as in the first snippet above) has the 'Parent' property which is an instance of the Library class - but to compile it would have to be defined as ITreeview.Parent

Would this work for you?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
  {
  class Program
    {
    static void Main(string[] args)
      {
      Library lib = new Library();
      ITreeview itv = (ITreeview)lib;
      Console.WriteLine(((Library)itv.Parent).Name);
      Console.ReadLine();
      }
    }

  public interface ITreeview
    {
    ITreeview Parent { get; }
    }

  public partial class Library
    {
    public Library Parent { get { return new Library(); } }
    public string Name { get { return "Hello World"; } }
    }

  public partial class Library : ITreeview
    {
    ITreeview ITreeview.Parent
      {
      get { return this.Parent; }
      }
    }
  }