Did you ever need to convert List(T1) to List(T2)? One example might be when implementing an interface. you might need to expose a collection of other interfaces (or maybe the same interface), But you usually hold the concrete type implementing the interface in the collection. Lets look at the following example:

We use collections all the time. Many times we have to expose them to users of our classes. Lets look at this simple tree node class:
class TreeNode
{
private List<TreeNode> children = new List<TreeNode>();

public IList<TreeNode> Children
{
[...]

Often we want to display a list of elements and allow the user to sort them in various ways. There are many ways to do it and I wanted to show you how to use a SortDescription Object. What is a SortDescription object you ask? It receives two parameters in its constructor: the first is [...]