C# 5.0 Programmer's Reference

Classes ❘ 247
Method
AssignJob
PrintInvoice
EstimatedCost
Purpose
Assigns the Job to an Employee
Prints an invoice for the Customer after the job is completed
Calculates and returns an estimated cost based on the customer’s
service contract type and EstimatedHours
The class could provide the events shown in the following table to keep the main program informed
about the job’s progress.
Event
Created
Assigned
Rejected
Canceled
Finished
Purpose
Occurs when the Job is first created
Occurs when the Job is assigned to an Employee
Occurs if an Employee refuses to do the job, perhaps because the
Employee doesn’t have the right skills or equipment to do the work
Occurs if the Customer cancels the job before it is started
Occurs when the job is completed
The class packages the data and behavior of some programming abstraction such as a Job, Employee,
Customer, Menu, SquashMatch, SoftwareProject, or anything else you might want to manipulate as
a single entity.
After you have defined a class, you can create as many instances of the class as you like. An instance
of the class is an object of the class type. For example, the Job class represents jobs in general. After
you have defined the Job class, you can make instances of the class to represent specific jobs. You
could create instances to represent building a brick wall, planting a tree, or repairing a telephone
switch. The process of creating an instance of a class is called instantiation.
There are a couple common analogies to describe instantiation. One compares the class to a blueprint.
After you define the class, you can use it to create any number of instances of the class, much as you
can use the blueprint to make any number of similar houses (instances).
The different houses have much in common. For example, the blueprint defines the number, size, and
relative placement of the houses’ rooms. These are analogous to features defined by the class that apply
to all the instances. (In the Job class example, all instances have a PrintInvoice method that prints
an invoice. Although exactly what is printed depends on the instance’s properties.)
The houses can also have differences such as different colors, front doors, and appliances. Those
correspond to the class’s property values. For example, a House class could define ExteriorColor
and ExteriorTrim properties that determine the color of each instance of the class.
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