Creating a hierarchical structure from a flat list

Question

I need to build an nth level hierarchical list from a flat list.

I tried to play around with the solution in this answer: https://stackoverflow.com/a/167168/2127 , but I was not able to get the correct results.

Here is my object structure:

public class Group { public string Id { get; set; } public string Name { get; set; } public string ParentId { get; set; } public List<Group> Children { get; set; } } public List<Group> BuildGroupHierarchy(List<Group> groups) { var lookup = groups.Where(x => x.ParentId != null).ToList().ToLookup(c => c.ParentId); List<string> idsToRemove = new List<string>(); foreach (var c in groups) { if (c.Children == null) c.Children = new List<Group>(); if (lookup.Contains(c.Id)) c.Children.AddRange(lookup[c.Id].ToList()); } return groups; } 

This is a virtual hierarchical structure present in flat:

• Group 1
  • Group 1-1
• Group 2
  • Group 2-1
    • Group 2-1-1
  • Group 2-2

But this is what I get, basically the second level is repeated:

• Group 1
  • Group 1-1
• Group 2
  • Group 2-1
    • Group 2-1-1
  • Group 2-2
• Group 2-1
  • Group 2-1-1