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

IDictionary> data;
var n = data.SelectMany(x => x.Value.Select(y => new { Topic = x.Key, Tag = y }))
  .GroupBy(x => x.Tag, x => x.Topic);