Recursive lambda expression to traverse a tree in C#

Question

Can someone show me how to implement a recursive lambda expression to traverse a tree structure in C#.

Answer 1

A proper solution, and indeed the idiomatic solution in many functional programming languages, would be the use of a fixed-point combinator. In a nutshell: a fixed-point combinator answers the question “how do I define an anonymous function to be recursive?”. But the solution is so nontrivial that whole articles are written to explain them.

A simple, pragmatic alternative is to “go back in time” to the antics of C: declaration before definition. Try the following (the “factorial” function):

Func fact = null;
fact = x => (x == 0) ? 1 : x * fact(x - 1);

Works like a charm.

Or, for a pre-order tree traversal on an object of class TreeNode which implements IEnumerable appropriately to go over its children:

Action> preorderTraverse = null;
preorderTraverse = (node, action) => {
    action(node);
    foreach (var child in node) preorderTraverse(child, action);
};