IEnumerable of IEnumerable of Int totalPatterns = new HashSet of Hashset of Int not wokring in 3.5

Question

 had a propblem in below in framework 3.5
IEnumerable of IEnumerable of Int totalPatterns = new HashSet of Hashset of Int
not complied.
if i type case as IEnumerable then compliend
but this is working framwork 4 without typecasting
My real propblem is
The above code is implemented in one dll(called service1)which has targetframework 3.5.
The two client which is used this service1 dll. one clinet target framework 3.5 and other 4. The client who is using framework 4 works perfectly. but client 3.5, totalpatterns object is null. since type cast not working
Please help
Gopal