Chapter 19: Generics

Classes are often described as cookie cutters for creating objects. You define a class, and then you can use it to make any number of objects that are instances of the class.

Similarly, a generic is like a cookie cutter for creating classes. You define a generic, and then you can use it to create any number of classes that have similar features.

For example, Visual Basic comes with a generic List class. You can use it to make lists of strings, lists of integers, lists of Employee objects, or lists of just about anything else.

This chapter explains generics. It shows how you define generics of your own and how you can use them.

Advantages of Generics

A generic class is tied to one or more specific data types. For example, you can build a list of OrderItem objects, a hash table containing PurchaseOrders identified by number, or a Queue that contains Customer objects.

Tying generics to specific data types gives them a few advantages over more traditional classes:

• Strong typing - Methods can take parameters and return values that have the class’s instance type. For example, a List(Of String) can only hold string values, and its Item method returns string values. This makes it more difficult to accidentally add the wrong type of object to the collection.

• IntelliSense - By providing strong typing, a class built from a generic lets Visual Studio provide IntelliSense. If you make a List(Of Employee), Visual Studio knows that the items in the collection are Employee objects, so it can give you appropriate IntelliSense.

• No boxing - Because the class manipulates objects with a specific data type, Visual Basic doesn’t need to convert items to and from the plain Object data type. For example, if a program stores TextBox controls in a normal collection, the program must convert the TextBox controls to and from the Object class when it adds and uses items in the collection. Avoiding these steps makes the code more efficient.

• Code reuse - You can use a generic class with more than one data type. For example, if you have built a generic PriorityQueue class, then you can make a PriorityQueue holding Employee, Customer, Order, or Objection objects. Without generics, you would need to build four separate classes to build strongly typed priority queues for each of these types of objects. Reusing this code makes it easier to write, test, debug, and maintain the code.