title: how to join two arrays with linq
body: <p>i am trying to combine two arrays with their relations but cant do exatly.
  I have a Posts Table on my database and in Posts table there are questions and answers records. answers are related with question on "relatedPostId" column.
  ex:</p>
  
  <pre><code>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
  </code></pre>
  
  <p>My code is like below</p>
  
  <pre><code>    var posts = DBConnection.GetComments(_post.TopicId).ToArray().OrderByDescending(p =&gt; p.CreateDate);
  
      var questions = posts.Where(p =&gt; p.RelatedPostId == null);
      var answers = posts.Where(p =&gt; 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 };
  </code></pre>
  
  <p>I want to list posts on a ListBox (lstCurrentTopic.Items.AddRange(...))
  Also i want to display answers at the end of each question like</p>
  
  <pre><code>Question1
   -&gt; Answer1 (relatedPostId is Qustion1.Id)
   -&gt; Answer2 (relatedPostId is Qustion1.Id)
  Qestion2
   -&gt;Answer3 (relatedPostId is Qustion2.Id)
   -&gt;Anser4 (relatedPostId is Qustion2.Id)
  </code></pre>
  
  <p>how can i add with this order to listbox</p>