Fnding children records in a self referencing collection

Lawrence Pond

12y5.2kLINQ

Using Linq to access I have a self-referencing collection in memory I would like to get all records that are beneath a given ID. I would also like to pass a depth level so it would not search children records after the level was reached. The level could help avoid a circular reference of self-referencing and endless loop

So here is my in memory object

ID NAME PARENTID

1 Bob null

2 Cody 1

3 Duke 1

4 Eric 2

5 Fred 4

6 Greg 5

Example if the user pass in the following ID=4 MAXLEVEL = 3 they would see the following results

4-Eric,

5-Fred,

6-Greg

The results should be flattened and not hierarchical. What would be the best approach using Linq thanks for you advise and help.

3 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes12y ago
Accepted answer
Try this recursive approach:

using System;
using System.Collections.Generic;
using System.Linq;

class Person
{
public int Id {get; set;}
public string Name {get; set;}
public int? ParentId {get; set;}

public Person(int id, string name, int? parentId)
{
Id = id;
Name = name;
ParentId = parentId;
}
}

class Program
{
static void Main()
{
List persons = new List
{
new Person(1, "Bob", null),
new Person(2, "Cody", 1),
new Person(3, "Duke", 1),
new Person(4, "Eric", 2),
new Person(5, "Fred", 4),
new Person(6, "Greg", 5)
};

var children = GetChildren(persons, 4, 3);

foreach(Person child in children)
{
Console.WriteLine("{0}-{1}", child.Id, child.Name);
}

Console.ReadKey();
}

static IEnumerable GetChildren(IEnumerable persons, int id, int

level)
{
if (level < 1) level = 1;
var parent = persons.Where(p => p.Id == id);
if (parent.Count() == 0) return Enumerable.Empty();
if (level == 1) return parent;
var children = GetChildren2(persons, parent.First().Id, level - 1);
if (children.Count() == 0) return parent;
return parent.Union(children).OrderBy(p => p.Id);
}

static IEnumerable GetChildren2(IEnumerable persons, int id, int

level)
{
var children = persons.Where(p => p.ParentId == id);
if (children.Count() == 0) return Enumerable.Empty();
if (level == 1) return children;
IEnumerable results = children;

foreach(Person c in children)
{
var grandChildren = GetChildren2(persons, c.Id, level - 1);
if(grandChildren.Count() > 0)
{
results = results.Union(grandChildren);
}
}

return results;
}
}

As expected the output is:

4-Eric
5-Fred
6-Greg

If you create a circular reference by making Cody's ParentId = 5, then the output is:

2-Cody
4-Eric
5-Fred
6-Greg

So, the level parameter does prevent an endless loop (and hence a stack overflow here) as you anticipated and the Union method prevents duplicates being returned.
Lawrence Pond12y ago
This is greatly appreciated, It does work as specified!
Biswa Pujarini Mohapatra12y ago
check out this threads
http://stackoverflow.com/questions/5377114/what-is-the-best-way-to-get-the-level-from-a-tree-data-structure-using-linq
http://stackoverflow.com/questions/12243658/depth-first-flattened-collection-of-an-object-hierarchy-using-linq
http://stackoverflow.com/questions/17968069/linq-sort-a-flat-list-based-on-childorder

hope it helps