select * from DoctorInfo doctor
where doctor.HosDepartId in
(select Id from HospitalDepartment hd
where hd.DepartmentId=5)

var a=from d in _entity.HospitalDepartment
where d.DepartmentId==5
select d;

List<int> lst=new List<int>();
foreach(var b in a)
{
lst.add(b);
}

var doc=from c in _entity.DoctorInfo
where lst.contains(c.HospitalId)
select c;