IEnumerable<Supplier> Suppliers = context.ExecuteQuery<Supplier>("SELECT * FROM fli WHERE liefnr = {0}", activeRowid);

                foreach (Supplier sup in Suppliers)
                {
                  if (sup.ContactPerson.Count > 0)
                  {
                    IEnumerable<ContactPerson> sp = (from con in sup.ContactPerson where con.kennung == 2 select con);
 
                  }
               }

[Association(Name="Supplier_ContactPerson", Storage="_ContactPerson", ThisKey="liefnr,1", OtherKey="feld, kennung")]

[Association(Name="Supplier_ContactPerson", Storage="_ContactPerson", ThisKey="liefnr", OtherKey="feld")]

[Association(Name="Supplier_ContactPerson", Storage="_ContactPerson", ThisKey="liefnr", OtherKey="feld")]
public EntitySet<ContactPerson> ContactPerson
{
get
{
                EntitySet<ContactPerson>  ContactPersons= new EntitySet<ContactPerson>();


                var suppliers = from c in this._ContactPerson
                           where c.kennung == 2
                           select c;

                foreach (var c in suppliers )
                    ContactPersons.Add(c);

                return ContactPersons;
}
set
{
this._ContactPerson.Assign(value);
}
}