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(...).