EDIT:
Ok I found why the int.ToString() in LINQtoEF fails, please read this post: Problem with converting int to string in Linq to entities
This works on my side :
List<string> materialTypes = (from u in result.Users
select u.LastName)
.Union(from u in result.Users
select SqlFunctions.StringConvert((double) u.UserId)).ToList();
On yours it should be like this:
IList<String> materialTypes = ((from tom in context.MaterialTypes
where tom.IsActive == true
select tom.Name)
.Union(from tom in context.MaterialTypes
where tom.IsActive == true
select SqlFunctions.StringConvert((double)tom.ID))).ToList();
Thanks, i've learnt something today :)