使用LINQ在List中查找顶级父级

Question

I have a List of user-defined objects that have an ID and a ParentID. The list looks something like this.

ParentID     ID
  123        345
  123        456
  456        567
  456        678
  678        789

I need a LINQ statement to find the top level parent; that is, all of the objects where the ParentID does not exist in as an ID (in this example, only 123).

Here is what I have so far and it is returning 567,678,789.

parentList = baseList.Where(b => !baseList.Select(o => o.ParentID).Distinct().Contains(b.ID)).ToList();

Answer 1

Your current query is trying to find all the items where their ID doesn't correspond to any other item's parent ID--in other words, you're finding all childless nodes.

What it sounds like you want is all the parentless nodes--those whose parent ID doesn't match any other item's ID.

var ids = new HashSet<int>(baseList.Select(o => o.ID));
var itemsWithNoParent = baseList.Where(o => !ids.Contains(o.ParentID))
    .ToList();

I'm using a HashSet<> to ensure reasonable .Contains() performance on large collections.

Answer 2

Also:

parentList = baseList
  .where(parent => !baseList.Any(possibleParent => possibleParent.ID == parent.ParentID))
  .ToList();

I use this a lot on small (less than 100,000) collections.

I might as well add this too; how to easily create a tree view from this:

public class Node
{
  public int Id { get; set; }
  public int ParentId { get; set; }
  public IEnumerable<Node> Nodes { get; set; }
  public Node ParentNode { get; set; }
}

IEnumerable<Node> nodes = .....

nodeTree = nodes.Select(n =>
{
  n.Nodes = nodes.Where(n2 => n2.ParentId == n.Id).ToList();
  n.ParentNode = nodes.FirstOrDefault(n2 => n2.Id == n.ParentId)
  return n;
})
.Where(n => n.ParentNode == null)
.ToList();