c# - LINQ union 2 tables -

i have table (t) has references 2 additional tables (a,b) (both references null-able, there still 1 filled in per row). need union data 3. if row in (t) references a, need data a, if b, data b. not problem, need project type, there collection of strings in table a, table b there no (assigning null or enumerable.empty<string>() or new list... produces error). ideas?

illustration :

(from t in context.t join in context.a on t.refa_id equals a.id select new projectiontype {    ...    collectionproperty = a.collection }).union( t in context.t join b in context.b on t.refb_id equals b.id select new projectiontype {    ...    collectionproperty = what_should_i_put_there??? // needed because of union }) 

edit: maybe simple question be: how project empty collection in linq entities?

edit2: exception: a type implements ienumerable 'system.collections.generic.list...' cannot initialized in linq entities query.

it seems, defining dummy variable outside query works