LEFT JOIN DirContract ON (условие-1)and(условие-2)or(условие-3))and(условие-4)...

from dirContractors in db.DirContractors

join dirContracts in db.DirContracts on dirContractors.DirContractorID equals dirContracts.DirContractorID into Contract1
from Contract in Contract1.DefaultIfEmpty()

where (Contract.DirContractDateEnd == null || Contract.DirContractDateEnd >= DateTime.Now)

                        join dirContracts in 
                        (
                            from dirContracts in db.DirContracts
                            where (dirContracts.DirContractDateEnd == null || dirContracts.DirContractDateEnd >= DateTime.Now)
                            select dirContracts
                        ) 
                        on dirContractors.DirContractorID equals dirContracts.DirContractorID into Contract1
                        from Contract in Contract1.DefaultIfEmpty()

where dirContracts.DirContractorID == dirContractors.DirContractorID