ASP.NET provides mechanisms for storing information for a single user session or across multiple sessions. This is done using the HttpSessionState and HttpApplicationState classes. The Page class has Application and Session attributes to provide access to current objects. The simple way to access them is as following:

if (Session["FirstName"] == null)
{
    LabelFirstName.Text = "FirstName";
}
else
{
    LabelFirstName.Text = (string)Session["FirstName"];
}
if (Session["LastName"] == null)
{
    LabelLastName.Text = "LastName";
}
else
{
    LabelLastName.Text = (string)Session["LastName"];
}

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
{
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