I was implementing a trie, which is a tree data structure, usually for storing strings for searching. Since it's a tree, it has a "Children" for holding child nodes.

But then I ran into a problem where simply calls to adding children to a collection (Line# 9) didn't work.

https://gist.github.com/dance2die/57e7aa68fcef7a7fe8cc983d0c922422

After 30 minutes of debugging, I was like...

Just what the hl happened?

TL;DR

Auto Property initialization creates a backing field while Expression Bodied property one does not

▬ Introduction ▬

According to Wikpedia, you can declare a trie like this (in Haskell).

https://gist.github.com/dance2die/7255d7236b1eed8d2dd287607cdaa88d

So I created a TrieNode class as shown below.

https://gist.github.com/dance2die/5b29a64f8f5ab7a880255b7e043e057f

If you are an astute reader, you might have already spotted the problem. Congratulations!

▬ Problem ▬

The complete source for building a trie, TrieBuilder is declared as below.

https://gist.github.com/dance2die/dd97d22fce289dd3359f307fb2c67364

Given a list of words passed to BuildTrie method, it populates a trie and returns an object instance.

Insert method simply checks for an existence of a character and maps current character to a node to the trie object instance , current.

This is where the problem occurred. current.Children.Add(...) wasn't adding  node object instance.

▬ Investigation ▬

Later on, I found out a StackOverflow answer explanating that declaring a property with => syntax (introduced in C# 6) does NOT create a backing field.

So my declaration below,

https://gist.github.com/dance2die/72ae994cff4f452330a732c53945d845

is equivalent to

https://gist.github.com/dance2die/e90747c077f43de460b13d0aeeccf512

returning a new array whenever Children property was accessed, thus not adding a new node to it.

▬ Solution ▬

The fix is simple. Declare Children with a backing field or use an auto property initialization syntax.

https://gist.github.com/dance2die/bca476ec22b64a031b8b6539572599df

Above declaration is equivalent to

https://gist.github.com/dance2die/8cee8a9abcd8fcec8ce6f5c35789705e

That was all it took to make me a happy camper ?.

▬ Takeaway ▬

Auto Property initialization creates a backing field while Expression Bodied property one does not.