linq - 如何编写一个 LINQ 查询来反转分层数据源的分组？

Question

如何编写一个 LINQ 查询，它采用分层源数据并对其进行转换，以便反转分组？

假设我有一个 Topic 对象列表，每个对象都包含一组表示该主题的元数据标签的标签。我需要的是编写一个 LINQ 查询来基本上翻转层次结构，以便我有一个标签列表，每个标签都有一个用该特定标签标记的主题集合。

Topic { Title = "Political Debate #1", Posted = 01/02/2008 }
   Tag { Name = "Contraversial", Color = "Red" }
   Tag { Name = "Politics", Color = "LightBlue" }
Topic { Title = "iPhone to support SiliverLight!", Posted = 02/23/2009 }
   Tag { Name = "BleedingEdge", Color = "LightBlue" }
   Tag { Name = "Contraversial", Color = "Red" }
   Tag { Name = ".NET", Color = "LightGreen" }
Topic { Title = "Fed Chairman admits guilt for causing second Great Depression", Posted = 06/15/2010 }
   Tag { Name = "Politics", Color = "LightBlue" }
   Tag { Name = "Contraversial", Color = "Red" }

我希望上面的数据看起来像下面的结果。

Tag { Name = "Contraversial", Color = "Red" }
    Topic { Title = "Political Debate #1", Posted = 01/02/2008 }
    Topic { Title = "iPhone to support SiliverLight!", Posted = 23/02/2009 }
    Topic { Title = "Fed Chairman admits guilt for causing second Great Depression", Posted = 06/15/2010 }
Tag { Name = "Politics", Color = "LightBlue" }
    Topic { Title = "Political Debate #1", Posted = 01/02/2008 }
    Topic { Title = "Fed Chairman admits guilt for causing second Great Depression", Posted = 06/15/2010 }
Tag { Name = ".NET", Color = "LightGreen" }
    Topic { Title = "iPhone to support SiliverLight!", Posted = 23/02/2009 }

您可以假设任何重复的数据段在引用上都是唯一的，因为它是内存中的单个实例，并且这些数据只有几个对同一对象的引用。此外，使用匿名类来生成投影的答案也是合理的，因为我意识到反转后类的形状可能略有不同。

更新:我在下面添加了设置示例数据的代码。我在 LinqPad 中玩弄已发布的答案和我自己的一些想法。

var tags = new[]
{
    new { Name = "Contraversial", Color = "Red" },
    new { Name = "Politics", Color = "LightBlue" },
    new { Name = ".NET", Color = "LightGreen" },
    new { Name = "BleedingEdge", Color = "LightBlue" }

};

var topics = new[]
{
    new 
    { 
        Title = "Political Debate #1", 
        Posted = DateTime.Parse("01/02/2008"), 
        Tags = (from t in tags where new []{"Contraversial", "Politics"}.Contains(t.Name) select t),
    },
    new 
    { 
        Title = "iPhone to support SiliverLight!", 
        Posted = DateTime.Parse("02/23/2009"), 
        Tags = (from t in tags where new []{"BleedingEdge", "Contraversial", ".NET", }.Contains(t.Name) select t),
    },
    new 
    { 
        Title = "Fed Chairman admits guilt for causing second Great Depression", 
        Posted = DateTime.Parse("06/15/2010"), 
        Tags = (from t in tags where new []{"Contraversial", "Politics"}.Contains(t.Name) select t),
    },
};

Answer 1

您正在寻找的是 枢。

Is it possible to Pivot data using LINQ?

This source 包含 Linq Pivot 扩展方法的 C# 代码:

public static class LinqExtensions 
{

    public static Dictionary<TFirstKey, Dictionary<TSecondKey, TValue>> Pivot<TSource, TFirstKey, TSecondKey, TValue>(this IEnumerable<TSource> source, Func<TSource, TFirstKey> firstKeySelector, Func<TSource, TSecondKey> secondKeySelector, Func<IEnumerable<TSource>, TValue> aggregate) 
    {
        var retVal = new Dictionary<TFirstKey, Dictionary<TSecondKey, TValue>>();

        var l = source.ToLookup(firstKeySelector);
        foreach (var item in l) 
        {
            var dict = new Dictionary<TSecondKey, TValue>();
            retVal.Add(item.Key, dict);
            var subdict = item.ToLookup(secondKeySelector);
            foreach (var subitem in subdict) 
            {
                dict.Add(subitem.Key, aggregate(subitem));
            }
        }

        return retVal;
    }

}