Beginning ASP.NET 4.5 in CSharp and VB Opsylum

Data Types and Variables x 155
Clear methods to remove one or all items from the collection. Just like arrays, they enable you to
iterate, or loop, over them to access the items in the collection.
When collections were first introduced in the .NET Framework 1.0, the ArrayList and Hashtable
became popular very quickly because they were so easy to use. The ArrayList enables you to add
arbitrary objects that are then stored in the order in which you add them, whereas the Hashtable
enables you to store objects referenced by a custom key. The main benefit of these collections over
their array cousins is that they can grow on demand. Unlike the previous example, where you
needed to resize the array to create room for the third role, the ArrayList grows dynamically when
required. The following example shows you how this works:
VB.NET
Dim roles As New ArrayList()
' Create a new ArrayList. You don't need
' to set its size explicitly
roles.Add("Administrators")
roles.Add("ContentManagers")
roles.Add("Members")
' Add the first role
' Add the second role
' Keep adding roles and the ArrayList
' grows as necessary
C#
ArrayList roles = new ArrayList(); // Create a new ArrayList. You don't need
// to set its size explicitly
roles.Add("Administrators"); // Add the first role
roles.Add("ContentManagers"); // Add the second role
roles.Add("Members");
// Keep adding roles and the ArrayList
// grows as necessary
Because this code now calls a method (Add) rather than assigning an item to a predefined index in
an array, you need parentheses (()) in both VB.NET and C#. The usage of methods is discussed
later in this chapter.
Although collections solve some of the problems that arrays have, they introduce a few problems of
their own. The biggest drawback of the ArrayList is that it isn’t strongly typed. What this means
is that you can add any object to the list using the Add method. This means that the ArrayList
could hold objects that are of different types at the same time. This may not seem to be a big deal
at first, but as soon as you start working with an ArrayList that contains multiple types of objects,
you’ll quickly see why this is problematic. Take the roles example again. With the array and the
ArrayList versions, the code simply added a few strings containing role names. You can then use
these three strings to, say, build up a drop-down list in a Web Form to enable a user to pick a role.
So far, so good. But what if one of the items in the list is not a string? What if another developer
accidentally wrote some code that adds a DropDownList control to the ArrayList? Because the
ArrayList accepts all objects, it won’t complain. However, your code will crash if it expects a
String, but gets a DropDownList control instead.
With .NET 2.0, Microsoft introduced a concept called generics. Generics are still strongly present
in version 4.5 of .NET, helping you overcome the problems that weakly typed collections like the
ArrayList introduced.