score:6

Accepted answer
entities.newinstance.subjects.where(x => alreadyselectedids.contains(x.id)).tolist();

should work, if alreadyselectedis is not null

you can do a null check inside or before your query :

entities.newinstance.subjects.where(x => alreadyselectedids == null 
                                         ? true // or false 
                                         : alreadyselectedids.contains(x.id)
                                    ).tolist();

(which can be rewritten, depending if you want all or nothing if alreadyselectedids is null)

//return all if null
x => alreadyselectedids == null || alreadyselectedids.contains(x.id)

or

//return nothing if null
x => alreadyselectedids != null  && alrreadyselectedids.contains(x.id)

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