var query = from a in data.A
            join b in data.B
            on a.bid equals b.id into ab
            from r1 in ab.DefaultIfEmpty()
            join c in data.C
            on r1.cid equals c.id into bc
            from r2 in bc.DefaultIfEmpty()
            join d in data.D
            on r2.did equals d.id into cd
            from r3 in cd.DefaultIfEmpty()
            join e in data.E
            on r3.eid equals e.id into ce
            from r4 in ce.DefaultIfEmpty()
            select new Signature
            {
                  Id = a.Id,
                  Name = r1.Name,
                  Pro = r2.Pro,
                  IpAdd = r3.IpAdd
            }; 
   IList<Signature> list = query.Take(pageSize).Skip((page - 1) * pageSize).ToList();//在这句报错
   return list;

var query = from a in data.A
            join b in data.B
            on a.bid equals b.id into ab
            from b in ab.DefaultIfEmpty()
            join c in data.C
            on b.cid equals c.id into bc
            from c in bc.DefaultIfEmpty()
            join d in data.D
            on c.did equals d.id into cd
            from d in cd.DefaultIfEmpty()
            join e in data.E
            on d.eid equals e.id into ce
            from e in ce.DefaultIfEmpty()
            select new Signature
            {
                  Id = a.Id,
                  Name = b==null?"":b.Name,
                  Pro = c==null?"":c.Pro,
                  IpAdd = d==null?"":d.IpAdd
            };