C#Linq左外连接

当ALLUSERDEVICE为null时，其他join fire在join ALLDEPORMATION行上显示异常。因为它是空的。如何修复此代码

join d in allDepartment on uD.DepartmentId equals d.DepartmentId into departmentJoin

--Ful代码是

var data = (from us in allUsers
                        join uR in allUserRoles on us.UserId equals uR.UserId into userRoleJoin
                        from uR in userRoleJoin.DefaultIfEmpty()
                        join r in allRoles on uR.RoleId equals r.RoleId 
                        join uD in allUserDepartment on us.UserId equals uD.NewUserId into userDepJoin 
                        from uD in userDepJoin.DefaultIfEmpty()
                        join d in allDepartment on uD.DepartmentId equals d.DepartmentId into departmentJoin
                        from d in departmentJoin.DefaultIfEmpty()

可能的重复可能的重复可能的重复