使用ToDictionary构建排序的字典 [英] Building a sorted dictionary using ToDictionary

问题描述

我不是C＃和LINQ的专家。

我有一个字典，我明白一个哈希表，也就是键不排序。

  dataBase = new Dictionary< string，Record>（）
  

记录是一个用户定义的类，的数据。

我发现一个有趣的例子，将这个字典转换成由LINQ排序的字典：

  var sortedDict =（从dataBase中的条目orderby entry.Key升序选择条目）
 .ToDictionary（pair => pair.Key，pair => pair.Value）; 
  

此代码正常工作。

问题：我发现 sortedDict c $ c> sortedDict 仍然是一个哈希表，一种类型：

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

我预计结果字典应该是一种 map 如C ++ STL一样，通常以（平衡）二叉树的形式实现，以维护键的顺序。但是，由此产生的字典仍然是哈希表。

如何 sortedDict 可以维护排序吗？哈希表不能保存键的顺序。除了典型的哈希表外，是否执行C＃的 Generic.Dictionary

解决方案

维护两个数据结构：一个保持枚举的插入顺序的平面数组，以及按键检索的哈希表。

如果在排序集上使用 ToDictionary（），枚举时将按顺序排列，但不会按顺序维护。任何新插入的项目将在枚举时添加到后面。

编辑：如果要依赖此行为，我建议您查看MSDN文档，以查看是否这是保证，或只是偶然的。

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?

解决方案

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.

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