c# - Building a sorted dictionary using ToDictionary

Question

I'm not an expert in C# and LINQ.

I have a Dictionary, which I understand a hash table, that is, keys are not sorted.

dataBase = new Dictionary<string, Record>()

Record is a user-defined class that holds a number of data for a given key string.

I found an interesting example that converts this Dictionary into a sorted dictionary by LINQ:

var sortedDict = (from entry in dataBase orderby entry.Key ascending select entry)
.ToDictionary(pair => pair.Key, pair => pair.Value);

This code works correctly. The resulting sortedDict is sorted by keys.

Question: I found that sortedDict is still a hash table, a type of:

System.Collections.Generic.Dictionary<string, Record>

I expected the resulting dictionary should be a sort of map as in C++ STL, which is generally implemented as a (balanced) binary tree to maintain the ordering of the keys. However, the resulting dictionary is still a hash table.

How sortedDict can maintain the ordering? A hash table can't hold the ordering of the keys. Is the implementation of C#'s Generic.Dictionary other than a typical hash table?

Answer 1

Dictionary maintains two data structures: a flat array that's kept in insertion order for enumeration, and the hash table for retrieval by key.

If you use ToDictionary() on a sorted set, it will be in order when enumerated, but it won't be maintained in order. Any newly inserted items will be added to the back when enumerating.

Edit: If you want to rely on this behaviour, I would recommend looking at the MSDN docs to see if this is guaranteed, or just incidental.