Question

I have a List of objects of type IGroup. These can be nested to an umlimited level, and I’m trying to group them after retrieving them from a database. I can’t get my head around how to recursively add all groups to the right parents. Any groups with null as a parent are top level groups. I can’t guarantee the order they come out of the database.

public interface IGroup {
  string ID { get; set; }
  string Name { get; set; }
  string ParentID { get; set; }
  IList<IGroup> Groups { get; set; }
  ...

So if I had a list of:

Group1: ID = g1, ParentID = null
Group1a: ID = g2, ParentID = g1
Group2: ID = g3, ParentID = null
Group1b: ID = g4, ParentID = g3
Group1bc: ID = g5, ParentID = g4

I’m trying to group them as:

|Group1
|--Group1a
|--Group1b
|--|
   |--Group1bc
|Group2

Anyone fancy a stab at grouping them recursively?

Answer 1

No need to be recursive. To wit:

var lookup = items.ToDictionary(g => g.ID); // items is IEnumerable<IGroup>
foreach (var item in items.Where(g => g.ParentID != null)) {
    lookup[item.ParentID].Groups.Add(item);
}
var parents = items.Where(g => g.ParentID == null);

Note that lookup[item.ParentID] will throw if there is no IGroup with the corresponding ParentID. You can handle this more gracefully with TryGetValue.

My implementation of IGroup:

public class Group : IGroup {
    public string ID { get; set; }
    public string Name { get; set; }
    public string ParentID { get; set; }
    public IList<IGroup> Groups { get; set; }
    public Group() {
        Groups = new List<IGroup>();
    }
}

My test items:

IEnumerable<IGroup> items = new List<IGroup>() {
    new Group() { ID = "g1", ParentID = null },
    new Group() { ID = "g2", ParentID = "g1" },
    new Group() { ID = "g3", ParentID = null },
    new Group() { ID = "g4", ParentID = "g3" },
    new Group() { ID = "g5", ParentID = "g4" },
    new Group() { ID = "g6", ParentID = "g5" }
};