How to flatten tree via LINQ?

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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(...).

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