Figure 2.2
Output of Listing 2.1.2 when viewed through a browser.

Adding Elements to a Hashtable
In Listing 2.1.2, we begin by creating an instance of the Hashtable class, htSalaries, on line 5. Next, we populate this hash table with our various employees and their respective salaries on lines 7 through 12. Note that the Add method, which adds an element to the Hashtable collection, takes two parameters: the first is an alphanumeric key by which the element will be referenced, and the second is the element itself, which needs to be of type Object.

In Listing 2.1.2, we are storing integer values in our Hashtable class. Of course we are not limited to storing just simple data types; rather, we can store any type of Object. As we'll see in an example later in this chapter, we can even create collections of collections (collections whose elements are also collections)!

Removing Elements from a Hashtable
The Hashtable class contains two methods to remove elements: Remove and Clear. Remove expects a single parameter, the alphanumeric key of the element to be removed. Line 25 demonstrates this behavior, removing the element referred to as "BillG" in the hash table. On line 34 we remove all the elements of the hash table via the Clear method. (Recall that all collection types contain a Clear method that demonstrates identical functionality.)

The Hashtable class contains two handy methods for determining whether a key or value exists. The first function, ContainsKey, takes a single parameter, the alphanumeric key to search for. If the key is found within the hash table, ContainsKey returns True. If the key is not found, ContainsKey returns False. In Listing 2.1.2, this method is used on line 24. The Hashtable class also supports a method called ContainsValue. This method accepts a single parameter of type Object and searches the hash table to see if any element contains that particular value. If it finds such an element, ContainsValue will return True; otherwise, it will return False. The ContainsKey and ContainsValue methods are used primarily for quickly determining whether a particular key or element exists in a Hashtable.

On line 24, a check was made to see if the key "BillG" existed before the Remove method was used. Checking to make sure an item exists before removing it is not required. If you use the Remove method to try to remove an element that does not exist (for example, if we had Remove("Homer") in Listing 2.2.1), no error or exception will occur.

The Keys and Values Collections
The Hashtable class exposes two collections as properties: Keys and Values. The Keys collection is, as its name suggests, a collection of all the alphanumeric key values in a Hashtable. Likewise, the Values collection is a collection of all the element values in a Hashtable. These two properties can be useful if you are only interested in, say, listing the various keys.

On line 30 in Listing 2.1.2, the DataSource property of the dgEmployees DataGrid is set to the Keys collection of the hySalaries Hashtable instance. Because the Keys property of the Hashtable class returns an ICollection interface, it can be bound to a DataGrid using data binding. For more information on data binding and using the DataGrid, refer to Chapter 7, "Data Presentation."

Working with the SortedList Class
So far we've examined two collections provided by the .NET Framework: the Hashtable class and the ArrayList class. Each of these collections indexes elements in a different manner. The ArrayList indexes each element numerically, whereas the Hashtable indexes each element with an alphanumeric key. The ArrayList orders each element sequentially, based on its numerical index; the Hashtable applies a seemingly random ordering (because the order is determined by a hashing algorithm).

What if you need a collection, though, that allow