Here is the query
from a in this._addresses where a.Street.Contains(street) || a.StreetAdditional.Contains(streetAdditional) select a).ToList(
I'd use the null-coalescing operator...
(from a in this._addresses where (a.Street ?? "").Contains(street) || (a.StreetAdditional ?? "").Contains(streetAdditional) select a).ToList()