集合是否有可能变得碎片化？
考虑在Dictionary中注册新成员的进入以及进入的时间：
Dictionary<string, DateTime> members = new Dictionary<string, DateTime>();
members.Add("Bob", DateTimeNow);
Thread.Sleep(1000);
members.Add("Joe", DateTimeNow);
Thread.Sleep(1000);
members.Add("Susan", DateTimeNow);
Thread.Sleep(1000);
// Now Joe exits
members.Remove("Joe");
// Then Mike enters
members.Add("Mike", DateTimeNow);
现在的问题是Mike在Dictionary中的位置。他是添加到 Dictionary 的末尾还是会填充 Susan 的空位置（如果我们使用 foreach 进行迭代或访问 字典 通过索引）？该行为是否始终得到保证？
Consider registering entrance of new members in a Dictionary and the time of entrance:
Dictionary<string, DateTime> members = new Dictionary<string, DateTime>();
members.Add("Bob", DateTimeNow);
Thread.Sleep(1000);
members.Add("Joe", DateTimeNow);
Thread.Sleep(1000);
members.Add("Susan", DateTimeNow);
Thread.Sleep(1000);
// Now Joe exits
members.Remove("Joe");
// Then Mike enters
members.Add("Mike", DateTimeNow);
Now the question is where's Mike location in Dictionary. Is he added to the end of Dictionary or he will fill the empty location of Susan (if we iterate with foreach or access the Dictionary via index)? Is the behavior guaranteed for all of the times?
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字典没有排序，因此如果您迭代键，您无法对它们返回的顺序进行任何推理。
正如 MSDN 网站告诉我们的，字典是作为哈希表实现的，并且：
如果您想迭代 a 中的键，则应该使用 SortedDictionary定义的顺序。
A Dictionary is not ordered, so you can't do any reasoning as to the order they're returned in if you iterate over the keys.
As the MSDN site tells us, Dictionary is implemented as a hash table, and:
You should use a SortedDictionary if you want to iterate over the keys in a defined order.
Dictionary 所代表的哈希表没有定义顺序的概念。您永远不能依赖特定顺序的项目。无法保证此行为。字典用于通过键快速访问给定的项目。
引自文档：
The notion of order is not defined for hashtable which what Dictionary represents. You can never rely on items being in a particular order. This behavior is not guaranteed. A Dictionary is used to quickly access a given item by key.
Quote from the documentation:
访问
字典是通过key完成的，而不是通过index完成的 - 所以它本身的问题意义不大。正如所有答案已经说明的那样 -
Dictionary没有顺序，并且通过Hashtable实现（Wiki 页面 - 花时间阅读它）字典本身可以扩展并且压缩取决于它维护的数据量和它自己的实现，但是当使用.NET这样的丰富框架时，你不需要不用管这个问题，它已经为你完成了。如果您感兴趣，可以阅读 WIKI 页面中的动态调整大小部分来了解其实现。Accessing a
Dictionaryis done viakeyand not viaindex- so the question it self is meaning less.As all answers already stated - a
Dictionaryhas no order, and is implemented via aHashtable(Wiki Page - Take the time to read it)The
Dictionaryit self can expand and compress depending on the volume of data it maintains and its own implementation, but when using a rich framework like.NETyour don't need to bother with that question, it is done for you. You can read theDynamic Resizingsection in the WIKI page to learn on the implementation if it interests you.