LINQ with subselect和groupby只获取列表中每个项目的最新版本

对于LINQ来说,我是新手......

我有一个IEnumerable通用列表,其中包含不同版本的答案(每个版本都有一个问题的FK).从这个列表中我只需要获得最新版本答案的字典.

一个非常简化的类图:

问题
-ID
-question
- ......其他属性

答
-ID
-Version
-QuestionID
-值
- ...其他属性

目前我有以下内容:

IEnumerable answers = GetAnswers();

IDictionary latestVersionAnswers = new Dictionary();
if (answers != null)
{
latestVersionAnswers = answers
      .OrderBy(e => e.ID)
      .GroupBy(e => e.Question.ID)
      .Select(g => new AnswerDTO
                        {
                             Version = g.Last().Version, // g.Select(e => e.Version).Max(), 
                             QuestionID = g.Key,
                             ID = g.Last().ID,
                             Value = g.Last().Value
                                 }).ToDictionary(c => c.QuestionID);
        }

虽然这在很大程度上起作用,但您可以很快发现它需要一些严肃的优化(并且有点脆弱,因为它取决于答案记录行顺序而不是"最大"逻辑).使用LINQ执行此操作的最佳方法是什么,或者最好只为每个循环执行多个操作？

如果我只需要版本(而不是ID,Value等)我就不需要OrderBy,因为我可以去g.Select(e => e.Version).Max()(或者我现在看到C#List <> GroupBy 2值的帖子,但这又会返回key/s和一个属性:Version).

最终,在这种特殊情况下,我更愿意"过滤"原始列表并返回原始答案项而不是涉及AnswerDTO.

任何指针或帮助将不胜感激!

怎么样......

   private void button1_Click(object sender, EventArgs e)
    {
      List list = GetAnswers();

      var dict = (from a in list
                 group a by a.QuestionID into grp
                 from g in grp
                 where g.Version == grp.Max(m => m.Version)
                 select new { id = g.QuestionID, q = g }).ToDictionary( o => o.id, o => o.q);

      StringBuilder sb = new StringBuilder();
      foreach (var elem in dict)
      {
        sb.AppendLine(elem.Key.ToString() + "-" + elem.Value.Version.ToString());
      }
      MessageBox.Show(sb.ToString());
    }

    private List GetAnswers()
    {
      List result = new List();
      result.Add(new Answer() { ID = 1, QuestionID = 1, Version = 1 });
      result.Add(new Answer() { ID = 2, QuestionID = 1, Version = 2 });
      result.Add(new Answer() { ID = 3, QuestionID = 1, Version = 3 });
      result.Add(new Answer() { ID = 4, QuestionID = 2, Version = 1 });
      result.Add(new Answer() { ID = 5, QuestionID = 2, Version = 2 });
      result.Add(new Answer() { ID = 6, QuestionID = 2, Version = 3 });
      result.Add(new Answer() { ID = 7, QuestionID = 3, Version = 1 });
      result.Add(new Answer() { ID = 8, QuestionID = 3, Version = 2 });
      result.Add(new Answer() { ID = 9, QuestionID = 3, Version = 3 });
      result.Add(new Answer() { ID = 10, QuestionID = 3, Version = 4 });
      return result;
    }

latestVersionAnswers = answers
  .GroupBy(e => e.Question.ID)
  .Select(g => g.OrderByDescending(e => e.Version).First())
  .ToDictionary(e => e.Question.ID);

或者,如果您更喜欢选择ToDictionary的重载:

latestVersionAnswers = answers
  .GroupBy(e => e.Question.ID)
  .ToDictionary(
    g => g.Key,
    g => g.OrderByDescending(e => e.Version).First()
  );