Get all children of a node with Linq in VB.net - vb.net

Question

I have Hierarchical datatable for filling TreeList in VB.Net like following:
ID ParentID Name
----------------------
1 NUll a
2 NUll b
3 2 c
4 1 d
5 3 e
6 5 f
7 6 g
8 5 h
My question:
How Can get list of all children for a node(ID) with Linq in vb.net?
Please help me.

Answer 1

Unfortunately I don't know VB good enough to give you a VB answer. I'll give the answer in C#. You'll probably understand the Idea. Maybe you can add your VB-translation?
You forgot to describe your Node class. I assume you do not only want the Children, but also the Grandchildren (etc).
class TreeNode
{
public int Id {get; set;}
public string Name {get; set;}
// every node has zero or more sub-nodes:
public IEnumerable<TreeNode> Nodes {get; set;}
}
You want your hierarchy to an unknown level deep. Hence you can't use the Standard LINQ functions. But you can easily extend LINQ to create your own. See Extension Function demystified
public static IEnumerable<TreeNode> AsTreeNodes(this IEnumerable<Person> persons)
{
// Top TreeNodes: all persons without a Parent:
return persons.AsTreeNodes((int?)null);
}
The actual function that returns the sequence of Nodes, of all persons with parentId, using recursion:
public static IEnumerable<TreeNode> AsTreeNodes(this IEnumerable<Person> persons, int? parentId)
{
// Top Nodes: all persons with parentId
var personsWithParentId = persons.Where(person.ParentId == parentId);
foreach (var person in personsWithParentId)
{
// every person will become one TreeNode with sub-nodes using recursion
TreeNode node = new TreeNode()
{
Id = person.Id,
Name = person.Name,
// get all my Children and Children's children:
Nodes = persons.ToNodeCollection(person.Id),
};
yield return node;
}
}
Note that I choose to return IEnumerable instead of List / Array. This way items will only be created when you really enumerate over them. So if you don't want to use the Node of the person with Id 1, all his children will not be created.
Usage: get the family hierarchy of all descendants of "George Washington":
IEnumerable<Person> allPersons = ...
IEnumerable<TreeNode> allFamilyHierarchies = allPersons.AsTreeNodes();
IEnumerable<TreeNode> washingtonFamily = allFamilyHierarchies
.Where(familyHierarchy => familyHierarchy.Name == "George Washington");
Note: until now, no TreeNode has been created, only the IEnumerable has been created.
The enumeration starts as soon as you do something that does not return IEnumerable,
like foreach, ToList, Any, FirstOrDefault.
Hence all non-Washington families will be ignored, no Top nodes, nor sub-nodes for them will be created.
If you look closely, then you'll see that to find all Children I have to enumerate over the complete collection, to find all Persons with a certain ParentId. This can be done way more efficiently if you first group all persons into groups with same ParentId and then put these groups into a Dictionary with the ParentId as Key. This way it will be very fast to find all Children of the parent with Id X:
var personsWithSameParentId = persons.GroupBy(person => person.ParentId);
var dictionary = personsWithSameParentId.ToDictionary(group => group.Key)
Every element in the dictionary has key equal to parentId and as elements all persons with this parentId.
TreeNode CreateNodeForPerson(Person person)
{
IEnumerable<Person> children = dictionary[person.Id];
IEnumerable<TreeNode> childNodes = children
.Select(child => CreateNodeforPerson(child));
return new TreeNode()
{
Id = person.Id,
Name = person.Name,
Nodes = childNodes,
};
}
You'll see the same recursion. But once you have the dictionary, you don't have to enumerate over the complete Person collection, you only access the children / children's children etc of the Person you are creating the nod for.