How do I write a LINQ query that inverts the grouping of a hierarchical data source?

Question

How would one write a LINQ query which takes a hierarchical source data and transforms it so that the grouping is inverted?

Say I have a list of Topic objects each of wh

Answer 1

What you're looking for is a Pivot.

Is it possible to Pivot data using LINQ?

This source contains C# code for a Linq Pivot extension method:

public static class LinqExtensions 
{

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

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

        return retVal;
    }

}