如何使用linq连接两个数组

Question

我试图将两个阵列与他们的关系结合起来,但不能做得很好.我的数据库和帖子表中有一个帖子表,有问题和答案记录.答案与"relatedPostId"栏中的问题有关.例如:

Posts (Table)
-------------
int Id (Field)
string Text (Field)
int RelatedPostId (Field)

question(record) : relatedPostId column is null
answer(record) : relatedPostId column is the value of question id

我的代码如下

    var posts = DBConnection.GetComments(_post.TopicId).ToArray().OrderByDescending(p => p.CreateDate);

    var questions = posts.Where(p => p.RelatedPostId == null);
    var answers = posts.Where(p => p.RelatedPostId != null);


    var result = from q in questions
                 join a in answers
                 on q.Id equals a.RelatedPostId
                 select  new { q = q, a = a };

我想列出ListBox上的帖子(lstCurrentTopic.Items.AddRange(...))另外我想在每个问题的末尾显示答案,如

Question1
 -> Answer1 (relatedPostId is Qustion1.Id)
 -> Answer2 (relatedPostId is Qustion1.Id)
Qestion2
 ->Answer3 (relatedPostId is Qustion2.Id)
 ->Anser4 (relatedPostId is Qustion2.Id)

如何将此订单添加到列表框中

Answer 1

这样的事情应该有效:

var result = from q in questions
             select new {
                q,
                answers = 
                    from a in answers
                    where q.Id == a.RelatedPostId
                    select a;
              }

上述方法适用于LINQ to Entities之类的东西,它可以转换为数据库可以优化的SQL语句.由于您正在使用LINQ到对象,因此如果您利用数据结构,您将获得更好的性能:

var answersByQuestionId = answers.ToLookup(a => a.RelatedPostId);
var result = from q in questions
             select new {
                q,
                answers = answersByQuestionId.Contains(q.Id)
                          ? answersByQuestionId[q.Id].AsEnumerable()
                          : Enumerable.Empty<Answer>()
              }