C# 容器可以分為無Generic時代 和 Generic時代。Generic是C# 2.0時的產物。
在System.Collections.Generic，也就是泛型命名空間出現以前。常用的是System.Collections 命名空間下的容器，包括ArrayList，Stack, SortedList, Queue等等。這些容器有一個共同特點：其存儲的元素都是object。這就意味着，比如ArrayList，它可以同時存放各種類型的數據，因為所有的類型都會轉化為object存儲，訪問元素時會自動提出數據。這樣的過程，我們稱其為“裝箱”和“拆箱”(boxing and unboxing)。
因此，System.Collections中的容器好處在於：不是嚴格類型的，可以放不同類型的值。壞處在於：因為對存放數據沒有類型要求，所以不是類型安全的；另外，裝箱和拆箱會是一個很耗費時間的工作。
arraylist vs List<> in c#
http://stackoverflow.com/questions/2309694/arraylist-vs-list-in-c-sharp
List<T> is a generic class. It supports storing values of a specific type without casting to or from object (which would have incurred boxing/unboxing overhead when T is a value type in the ArrayList case). ArrayList simply stores object references. As a generic collection, it implements the generic IEnumerable<T> interface and can be used easily in LINQ (without requiring any Cast or OfType call).
ArrayList belongs to the days that C# didn't have generics. It's deprecated in favor of List<T>. You shouldn't use ArrayList in new code that targets .NET >= 2.0 unless you have to interface with an old API that uses it.
引申出generic。
引申出LINQ，
List<T> 等屬於 Generics，Arraylist等屬於早期的容器
The List<T> class is the generic equivalent of the ArrayList class. It implements the IList<T> generic interface by using an array whose size is dynamically increased as required.
我們都知道List<T>的Add是O(1)的操作，但之所以它是O(1)，是因為它的“擴容”操作被均攤了（amortized），但每次擴容時其實還是需要復制所有元素，次數越少越好
http://blog.zhaojie.me/2013/01/think-in-detail-why-its-o-1.html
In deciding whether to use the List<T> or ArrayList class, both of which have similar functionality, remember that the List<T> class performs better in most cases and is type safe. If a reference type is used for type T of the List<T> class, the behavior of the two classes is identical. However, if a value type is used for type T, you need to consider implementation and boxing issues.
來自http://msdn.microsoft.com/en-us/library/6sh2ey19(v=vs.110).aspx
衍生出裝箱和拆箱。。。
Benefits of Generics
http://msdn.microsoft.com/en-us/library/b5bx6xee.aspx
LINQ Query
http://www.cnblogs.com/greenerycn/archive/2010/10/22/linq_intro.html