Question

我已经做了很多搜索，但是在涉及分组时，我找不到很多可以将LINQ查询语法转换为方法语法的好例子。

例如，这在方法语法中会是什么？

List<ChapterDTO> chapters = new List<ChapterDTO>(); // Prepopulated
List<SubChapterDTO> subChapters = new List<SubChapterDTO>(); // Prepopulated
var result = from c in chapters
             join sc in subChapters on c.ID equals sc.ChapterID
             orderby c.Number
             group sc by c.DisplayName into chapterGroup
             select new Grouping<string, SubChapterDTO>(chapterGroup.Key, chapterGroup);

我尝试了这个，但结果却不一样：

var result = chapters.Join(subChapters, c => c.ID, sc => sc.ChapterID, (c, sc) => new { Chapter = c, SubChapter = sc })
                     .OrderBy(x => x.Chapter.Number)
                     .GroupBy(x => x.Chapter.DisplayName);

如果我将它添加到最后，它将无法编译：

.Select(x => new Grouping<string, SubChapterDTO>(x.Key, x);

"cannot convert from 'System.Linq.IGrouping<string, <anonymous type: ChapterDTO Chapter, SubChapterDTO SubChapter>>' to 'System.Collections.Generic.IEnumerable<SubChapterDTO>'"

我错过了什么？

Answer 1

在GroupBy中使用不同的elementSelector

var result = chapters.Join(subChapters, c => c.ID, sc => sc.ChapterID, (c, sc) => new { Chapter = c, SubChapter = sc })
         .OrderBy(x => x.Chapter.Number)
         .GroupBy(x => x.Chapter.DisplayName, x => x.SubChapter)
         .Select(x => new Grouping<string, SubChapterDTO>(x.Key, x));

Answer 2

您可以尝试下面的代码

   var result = chapters.Join(subChapters, c => c.ID, sc => sc.ChapterID, (c, sc) => new { Chapter = c, SubChapter = sc })
                         .OrderBy(x => x.Chapter.Number)
                         .GroupBy(x => x.Chapter.DisplayName)
                         .SelectMany(gr => gr)
                         .ToList<string, SubChapterDTO>(x.Key, x);

将LINQ查询语法转换为具有分组的方法语法

2 个答案: