为什么这在EF中不起作用

Question

有人可以解释为什么这不起作用。

private void AdditionalLoad(List<PersonTypePerson> ptps)
    {
        using(var _context = new SPIS_Entities())
        {
            var m = (from ptp in ptps.Where(xx => xx.PersonID == _Person.ID)//_context.PersonTypePerson //
                     join pt in _context.PersonType on ptp.PersonTypeID equals pt.ID
                     join pta in _context.PersonTypeAttriubute.Where(p => p.active) on pt.ID equals pta.PersontypeID
                     select new { persontypeatribute = pta, availableatribute = pta.Attribute }).ToList();


            var x = from ii in _context.InformationItem.Where(p=>p.PersonID ==_Person.ID)
                    join pta in _context.PersonTypeAttriubute.Where(p => p.active) on ii.PersonTypeAtributeID equals pta.ID
                    select new { persontype = pta.PersonType, attribute = pta.Attribute, information = ii };


            var z = (from all in m
                     join fill in x on all.persontypeatribute.AttributeID equals fill.attribute.ID into ps
                     from fill in ps.DefaultIfEmpty()
                     select new
                     {
                         persontype = all.persontypeatribute.PersonType, //fill.persontype,
                         available = all.availableatribute,
                         attribute = fill.attribute,
                         information = fill.information
                     }).ToList();

        }
    }

最有趣的是我从“m”和“x”获得查询结果。当涉及到使左连接（“z”）的点时，它会中断并抛出nullException，对象引用未设置为对象的实例。