在另一个linq查询中使用结果linq查询

Question

Good day!

Tell me whether it is possible to use the result of a Linq query in another Linq query? For example:

    List<Member> memberList = db.Members.Where(m=>m.Year == 20013).ToList();
var result = (from members in memberList
             from documents in Archive.Where(d=>d.MemberId = members.Id).DefaultIfEmpty()
             select new ArchiveRecord
             {
                member = members,
                documentId = documentsId
             }).ToList();

At the same time the implementation of the second request falls with the error (There is already an open DataReader associated with this Command which must be closed first).

Please tell me where I made mistakes.

Thank you!

Answer 1

I don't know how to make nested linq query but i have solution for you. Try this:

List<Member> memberList = db.Members.Where(m => m.Year == 20013).ToList();
var membersIds = memberList.Select(m => m.Id);
var result = (from documents in Archive.Where(d => membersIds.Contains(d.MemberId)).DefultIfEmpty()
             select new ArchiveRecord
             {
                 member = members,
                 documentId = documentsId
              }).ToList();

Answer 2

在您的连接字符串中启用多个活动结果集（“ MultipleActiveResultSets = True”）： https : //msdn.microsoft.com/zh-cn/library/h32h3abf(v =vs.110) .aspx

Answer 3

I believe this should work (Single query):

var memberList = db.Members.Where(m=>m.Year == 20013);
var result = (from members in memberList
         from documents in Archive.Where(d=>d.MemberId = members.Id).DefaultIfEmpty()
         select new ArchiveRecord
         {
            member = members,
            documentId = documentsId
         }).ToList();

This definitely should work (Single query):

var result = (from members in db.Members
         from documents in Archive.Where(d=>d.MemberId = members.Id).DefaultIfEmpty()
         where members.year == 20013
         select new ArchiveRecord
         {
            member = members,
            documentId = documentsId
         }).ToList();

and this (Two queries):

var membersIds = db.Members
  .Where(m => m.Year == 20013)
  .Select(m => m.Id)
  .ToList();

var result = (from documents in Archive.Where(d => membersIds.Contains(d.MemberId)).DefultIfEmpty()
             select new ArchiveRecord
             {
                 member = members,
                 documentId = documentsId
              }).ToList();

and this assuming you have created a navigation property on members to archive, and it is a 1-many relationship (Single query):

var result=db.Archives
  .Where(a=>a.Member.Year==20013)
  .Select(a=> new ArchiveRecord {
    member=a.Member,
    documentId=a.documentsId
  }).ToList();