Expression Bodied Collection Property Initialization Gotcha in C#

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September 16, 2017
💫 Originally posted here. Broken? Let me know ~

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.

After 30 minutes of debugging, I was like…

Just what the h**l 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).

So I created a TrieNode class as shown below.

*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.

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,

is equivalent to

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.

Above declaration is equivalent to

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.