通过GroupBy查找最大值时抛出了“Sequence contains no elements”异常

Question

通过GroupBy查找最大值时抛出了“Sequence contains no elements”异常

3

我有以下语句，使用 GroupBy 计算最大值，并使用 Any() 来检查是否有任何记录，但是这个查询仍然返回 Sequence contains no elements exception 错误。有什么想法吗？

baseQuery = baseQuery.Where(s => s.LearningActions.SelectMany(la => la.ProgressUpdates)
                     .GroupBy(d => d.LearningActionId)
                     .Select(a => a.Any() ? new { Max = a.Max(d => d.Progress) } : new { Max = 0})
                     .Average(d => d.Max) < averageProgress.GetValueOrDefault());

以防万一，这里是 baseQuery。

 IQueryable<Submission> baseQuery = _context.Submissions                 
             .Where(s => s.ReviewRoundId == reviewRoundId);

我也尝试了以下替代方案：

baseQuery = baseQuery.Where(s => s.LearningActions.SelectMany(la => la.ProgressUpdates)
                      .GroupBy(d => d.LearningActionId)
                      .Select(a => new { Max = (int?) a.Max(d => d.Progress) })
                      .Average(a => a.Max.GetValueOrDefault()) < averageProgress.GetValueOrDefault());

但它会引发以下错误：

查询源（来自 [a] 中的 LearningActionProgress d）已经与表达式相关联。

更新

以下代码可行：

 baseQuery = baseQuery.Where(s => s.LearningActions.SelectMany(la => la.ProgressUpdates)
                                                      .GroupBy(d => d.LearningActionId)
                                                      .Select(a => a.Max(d => d.Progress)).Any() ?
                                                      s.LearningActions.SelectMany(la => la.ProgressUpdates)
                                                      .GroupBy(d => d.LearningActionId)
                                                      .Select(a => a.Max(d => d.Progress)).Average() > averageProgress.GetValueOrDefault() : false); // Here a can be null or empty

我不得不做这个检查 s.LearningActions.SelectMany(la => la.ProgressUpdates).GroupBy(d => d.LearningActionId).Select(a => a.Max(d => d.Progress)).Any()

但是，我觉得有点冗长，有什么更好的方式可以包含这个 check 吗？

- renakre

2个回答

1

你能为使用DefaultIfEmpty创建一个合理的默认对象吗？ MSDN: DefaultIfEmpty

baseQuery = baseQuery.Where(s => s.LearningActions.SelectMany(la => la.ProgressUpdates)
                     .GroupBy(d => d.LearningActionId)
                     .Select(a => a.Any() ? new { Max = a.Max(d => d.Progress) } : new { Max = 0})
                     .DefaultIfEmpty(defaultItem)
                     .Average(d => d.Max) < averageProgress.GetValueOrDefault());

- Mihaeru

谢谢您的回答！您能举个例子来说明“您是否可以创建一个合理的默认对象”吗？仅使用DefaultIfEmpty()就可行吗？ - renakre

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- kara · Accepted Answer

您的问题在于 .Average(a => a.Max)。a 可能为 null 或为空。您需要进行检查:

IEnumerable<Submission> baseQuery = new List<Submission>()
    {
        new Submission()
            {
                LearningActions = new List<LearningAction>()
                    {
                        //new LearningAction() { ProgressUpdates = null }, // will throw nullRef
                        new LearningAction() { ProgressUpdates = new List<ProgressUpdate>() }, // this is your problem
                    },
            }
    };

int? averageProgress = 100;

baseQuery = baseQuery.Where(
    s => s.LearningActions.SelectMany(la => la.ProgressUpdates)
         .GroupBy(d => d.LearningActionId)
         .Select(a => a.Any() ? new { Max = a.Max(d => d.Progress) } : new { Max = 0 })
         .Average(a => a?.Max) < averageProgress.GetValueOrDefault()); // Here a can be null or empty
         // For Expressions use Any again:
         // .Average(a => a.Any() ? a : 0) < averageProgress.GetValueOrDefault());

Console.WriteLine(string.Join("\n\n", baseQuery));

如果您想使用此代码，可以将其缩短一点。您不需要匿名类：

baseQuery = baseQuery.Where(
    s => s.LearningActions.SelectMany(la => la.ProgressUpdates)
         .GroupBy(d => d.LearningActionId)
         .Select(a => a?.Max(d => d.Progress)) // select the "int?"
         .Average() < averageProgress.GetValueOrDefault()); // Average can be performed on the resulting IEnumerable<int?>