Put simply, a SortedList is a Hashtable which is sorted by the keys. Like a Hashtable, it is accessible by both its keys and values. The API documentation describes a SortedList as a hybrid between a Hashtable and an Array. It behaves like a Hashtable when accessed via a key. It behaves like an Array when accessed via its GetByIndex method.
If you do not need to access elements in a SortedList via its indices, then use a Hashtable rather than a SortedList because the sorting has a performance implication, especially if you do not need that functionality.
The program below demonstrates how a SortedList can be used. The output is interspersed with the code to show the outputs of the different sections.
1: using System; 2: using ...