How to flatten tree via LINQ?

Question

So I have simple tree:

class MyNode
{
 public MyNode Parent;
 public IEnumerable<MyNode> Elements;
 int group = 1;
}

I have a IEnumerable<MyNode>. I want to get a list of all MyNode (including inner node objects (Elements)) as one flat list Where group == 1. How to do such thing via LINQ?

Answer 1

You can flatten a tree like this:

IEnumerable<MyNode> Flatten(IEnumerable<MyNode> e) =>
    e.SelectMany(c => Flatten(c.Elements)).Concat(new[] { e });

You can then filter by group using Where(...).

To earn some "points for style", convert Flatten to an extension function in a static class.

public static IEnumerable<MyNode> Flatten(this IEnumerable<MyNode> e) =>
    e.SelectMany(c => c.Elements.Flatten()).Concat(e);

To earn more points for "even better style", convert Flatten to a generic extension method that takes a tree and a function that produces descendants from a node:

public static IEnumerable<T> Flatten<T>(
    this IEnumerable<T> e
,   Func<T,IEnumerable<T>> f
) => e.SelectMany(c => f(c).Flatten(f)).Concat(e);

Call this function like this:

IEnumerable<MyNode> tree = ....
var res = tree.Flatten(node => node.Elements);

If you would prefer flattening in pre-order rather than in post-order, switch around the sides of the Concat(...).