Here is my code snippet:
var joinedList = List1.List.Where(x => x != null) .Union(List2.List.Where(x=> x!= null).Union(List3.List.Where(x => x!= null))).ToList();
var joinedList = new List<T>(); if (List1.List != null) joinedList = joinedList.Union(List1.List.Where(x=>x!=null); if (List2.List != null) joinedList = joinedList.Union(List2.List.Where(x=>x!=null); if (List3.List != null) joinedList = joinedList.Union(List3.List.Where(x=>x!=null);