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4.5 Instance Variables and Properties

In Chapter 3, we declared all of an application's variables in the application's Main method. Variables declared in the body of a method are known as local variables and can be used only in that method. When a method terminates, the values of its local variables are lost. Recall from Section 4.2 that an object has attributes that are carried with it as it's used in an application. Such attributes exist before a method is called on an object and after the method completes execution.

Attributes are represented as variables in a class declaration. Such variables are called fields and are declared inside a class declaration but outside the bodies of the class's method declarations. When each object of a class maintains its own copy of an attribute, the field that represents the attribute is also known as an instance variable—each object (instance) of the class has a separate instance of the variable. In Chapter 10, we discuss another type of field called a static variable, where all objects of the same class share one variable.

A class normally contains one or more properties that manipulate the attributes that belong to a particular object of the class. The example in this section demonstrates a GradeBook class that contains a courseName instance variable to represent a particular GradeBook object's course name, and a CourseName property to manipulate courseName.

GradeBook Class with an Instance Variable and a Property

In our next application (Figs. 4.7–4.8), class GradeBook (Fig. 4.7) maintains the course name as an instance variable so that it can be used or modified at any time during an application's execution. The class also contains one method—DisplayMessage (lines 24–30)—and one property—CourseName (line 11–21). Recall from Chapter 2 that properties are used to manipulate an object's attributes. For example, in that chapter, we used a Label's Text property to specify the text to display on the Label. In this example, we use a property in code rather than in the Properties window of the IDE. To do this, we first declare a property as a member of the GradeBook class. As you'll soon see, the GradeBook's CourseName property can be used to store a course name in a GradeBook (in instance variable courseName) or retrieve the GradeBook's course name (from instance variable course-Name). Method DisplayMessage—which now specifies no parameters—still displays a welcome message that includes the course name. However, the method now uses the CourseName property to obtain the course name from instance variable courseName.

Fig 4.7. GradeBook class that contains a private instance variable, courseName, and a public property to get and set its value.

1  // Fig. 4.7: GradeBook.cs
2  // GradeBook class that contains a private instance variable, courseName,
3  // and a public property to get and set its value.
4  using System;
6  public class GradeBook
7  {
8     private string courseName; // course name for this GradeBook
10     // property to get and set the course name
11     public string CourseName
12     {                                         
13        get
14        {                                      
15           return courseName;
16        }  // end get                          
17        set
18        {                                      
19           courseName = value;           
20        }  // end set                          
21     } // end property CourseName                
23     // display a welcome message to the GradeBook user
24     public void DisplayMessage()
25     {
26        // use property CourseName to get the
27        // name of the course that this GradeBook represents
28        Console.WriteLine( "Welcome to the grade book for\n{0}!",
29           CourseName ); // display property CourseName
30     }  // end method DisplayMessage
31  }  // end class GradeBook

Fig 4.8. Create and manipulate a GradeBook object.

1  // Fig. 4.8: GradeBookTest.cs
2  // Create and manipulate a GradeBook object.
3  using System;
5  public class GradeBookTest
6  {
7     // Main method begins program execution
8     public static void Main( string[] args )
9     {
10        // create a GradeBook object and assign it to myGradeBook
11        GradeBook myGradeBook = new GradeBook();
13        // display initial value of CourseName
14        Console.WriteLine( "Initial course name is: '{0}'\n",
15        myGradeBook.CourseName );
17        // prompt for and read course name
18        Console.WriteLine( "Please enter the course name:" );
19        myGradeBook.CourseName = Console.ReadLine(); // set CourseName
20        Console.WriteLine(); // output a blank line
22        // display welcome message after specifying course name
23        myGradeBook.DisplayMessage();
24     }  // end Main
25  }  // end class GradeBookTest

A typical instructor teaches more than one course, each with its own course name. Line 8 declares courseName as a variable of type string. Line 8 is a declaration for an instance variable, because the variable is declared in the class's body (lines 7–31) but outside the bodies of the class's method (lines 24–30) and property (lines 11–21). Every instance (i.e., object) of class GradeBook contains one copy of each instance variable. For example, if there are two GradeBook objects, each object has its own copy of courseName. All the methods and properties of class GradeBook can directly manipulate its instance variable courseName, but it's considered good practice for methods of a class to use that class's properties to manipulate instance variables (as we do in line 29 of method DisplayMessage). The software engineering reasons for this will soon become clear.

Access Modifiers public and private

Most instance-variable declarations are preceded with the keyword private (as in line 8). Like public, keyword private is an access modifier. Variables, properties or methods declared with access modifier private are accessible only to properties and methods of the class in which they're declared. Thus, variable courseName can be used only in property CourseName and method DisplayMessage of class GradeBook.

Declaring instance variables with access modifier private is known as information hiding. When an application creates (instantiates) an object of class GradeBook, variable courseName is encapsulated (hidden) in the object and can be accessed only by methods and properties of the object's class.

Setting and Getting the Values of private Instance Variables

How can we allow a program to manipulate a class's private instance variables but ensure that they remain in a valid state? We need to provide controlled ways for programmers to "get" (i.e., retrieve) the value in an instance variable and "set" (i.e., modify) the value in an instance variable. Although you can define methods like GetCourseName and SetCourse-Name, C# properties provide a more elegant solution. Next, we show how to declare and use properties.

GradeBook Class with a Property

The GradeBook class's CourseName property declaration is located in lines 11–21 of Fig. 4.7. The property begins in line 11 with an access modifier (in this case, public), followed by the type that the property represents (string) and the property's name (Course-Name). Properties use the same naming conventions as methods and classes.

Properties contain accessors that handle the details of returning and modifying data. A property declaration can contain a get accessor, a set accessor or both. The get accessor (lines 13–16) enables a client to read the value of private instance variable courseName; the set accessor (lines 17–20) enables a client to modify courseName.

After defining a property, you can use it like a variable in your code. For example, you can assign a value to a property using the = (assignment) operator. This executes the code in the property's set accessor to set the value of the corresponding instance variable. Similarly, referencing the property to use its value (for example, to display it on the screen) executes the code in the property's get accessor to obtain the corresponding instance variable's value. We show how to use properties shortly. By convention, we name each property with the capitalized name of the instance variable that it manipulates (e.g., CourseName is the property that represents instance variable courseName)—C# is case sensitive, so these are distinct identifiers.

get and set Accessors

Let us look more closely at property CourseName's get and set accessors (Fig. 4.7). The get accessor (lines 13–16) begins with the identifier get and its body is delimited by braces. The accessor's body contains a return statement, which consists of the keyword return followed by an expression. The expression's value is returned to the client code that uses the property. In this example, the value of courseName is returned when the property CourseName is referenced. For example, in the following statement

            string theCourseName = gradeBook.CourseName;

the expression gradeBook.CourseName (where gradeBook is an object of class GradeBook) executes property CourseName's get accessor, which returns the value of instance variable courseName. That value is then stored in variable theCourseName. Property CourseName can be used as simply as if it were an instance variable. The property notation allows the client to think of the property as the underlying data. Again, the client cannot directly manipulate instance variable courseName because it's private.

The set accessor (lines 17–20) begins with the identifier set and its body is delimited by braces. When the property CourseName appears in an assignment statement, as in

           gradeBook.CourseName = "CS100 Introduction to Computers";

the text "CS100 Introduction to Computers" is assigned to the set accessor's contextual keyword named value and the set accessor executes. Note that value is implicitly declared and initialized in the set accessor—it's a compilation error to declare a local variable value in this body. Line 19 stores the contents of value in instance variable courseName. A set accessor does not return any data when it completes its task.

The statements inside the property in lines 15 and 19 (Fig. 4.7) each access course-Name even though it was declared outside the property. We can use instance variable courseName in the methods and properties of class GradeBook, because courseName is an instance variable of the class.

Using Property CourseName in Method DisplayMessage

Method DisplayMessage (lines 24–30 of Fig. 4.7) does not receive any parameters. Lines 28–29 output a welcome message that includes the value of instance variable courseName. We do not reference courseName directly. Instead, we access property CourseName (line 29), which executes the property's get accessor, returning the value of courseName.

GradeBookTest Class That Demonstrates Class GradeBook

Class GradeBookTest (Fig. 4.8) creates a GradeBook object and demonstrates property CourseName. Line 11 creates a GradeBook object and assigns it to local variable myGrade-Book. Lines 14–15 display the initial course name using the object's CourseName property—this executes the property's get accessor, which returns the value of courseName.

The first line of the output shows an empty name (marked by single quotes, ''). Unlike local variables, which are not automatically initialized, every field has a default initial value—a value provided by C# when you do not specify the initial value. Thus, fields are not required to be explicitly initialized before they're used in an application—unless they must be initialized to values other than their default values. The default value for an instance variable of type string (like courseName) is null. When you display a string variable that contains the value null, no text is displayed on the screen.

Line 18 prompts the user to enter a course name. Line 19 assigns the course name entered by the user to object myGradeBook's CourseName property. When a value is assigned to CourseName, the value specified (which is returned by ReadLine in this case) is assigned to implicit parameter value of CourseName's set accessor (lines 17–20, Fig. 4.7). Then parameter value is assigned by the set accessor to instance variable courseName (line 19 of Fig. 4.7). Line 20 (Fig. 4.8) displays a blank line, then line 23 calls myGradeBook's DisplayMessage method to display the welcome message containing the course name.

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