Visual Basic.NET How to Program (PDF)

190 Procedures Chapter 6
In the Function header, the return-type indicates the data type of the result returned from
the Function to its caller. The statement
Return expression
can occur anywhere in a Function procedure body and returns the value of expression
to the caller. If necessary, Visual Basic attempts to convert the expression to the Function
procedure’s return-type. Functions Return exactly one value. When a Return
statement is executed, control returns immediately to the point at which that
procedure was invoked.
Common Programming Error 6.4
If the expression in a Return statement cannot be converted to the Function procedure’s
return-type, a runtime error is generated. 6.4
Common Programming Error 6.5
Failure to return a value from a Function procedure (e.g., by forgetting to provide a Return
statement) causes the procedure to return the default value for the return-type, often
producing incorrect output. 6.5
6.5 Methods
A method is any procedure that is contained within a class. We have already presented several
FCL methods (i.e., methods contained in classes that are part of the FCL). Programmers
also can define custom methods in programmer-defined classes, such as a class used
to define a Windows application. The Windows application in Fig. 6.4 uses two methods
to calculate the largest of three Doubles.
1 ' Fig. 6.4: Maximum.vb
2 ' Program finds the maximum of three numbers input.
3
4 Public Class FrmMaximum
5 Inherits System.Windows.Forms.Form
6
7 ' prompts for three inputs
8 Friend WithEvents lblOne As System.Windows.Forms.Label
9 Friend WithEvents lblTwo As System.Windows.Forms.Label
10 Friend WithEvents lblThree As System.Windows.Forms.Label
11
12 ' displays result
13 Friend WithEvents lblMaximum As System.Windows.Forms.Label
14
15 ' read three numbers
16 Friend WithEvents txtFirst As System.Windows.Forms.TextBox
17 Friend WithEvents txtSecond As System.Windows.Forms.TextBox
18 Friend WithEvents txtThird As System.Windows.Forms.TextBox
19
20 ' reads inputs and calculate results
21 Friend WithEvents cmdMaximum As System.Windows.Forms.Button
22
Fig. Fig. 6.4 6.4 Method that determines the largest of three numbers (part 1 of 2).