Visual Basic.NET How to Program (PDF)

466 Exception Handling Chapter 11
12
13 ' constructor for customizing error message
14 Public Sub New(ByVal messageValue As String)
15 MyBase.New(messageValue)
16 End Sub ' New
17
18 ' constructor for customizing error message and specifying
19 ' InnerException object
20 Public Sub New(ByVal messageValue As String, _
21 ByVal inner As Exception)
22
23 MyBase.New(messageValue, inner)
24 End Sub ' New
25
26 End Class ' NegativeNumberException
Fig. 11.4 ApplicationException derived class thrown when a program
performs an illegal operation on a negative number (part 2 of 2).
1 ' Fig. 11.5: SquareRootTest.vb
2 ' Demonstrating a programmer-defined exception class.
3
4 Imports System.Windows.Forms
5
6 Public Class FrmSquareRoot
7 Inherits Form
8
9 ' Label for showing square root
10 Friend WithEvents lblOutput As Label
11 Friend WithEvents lblInput As Label
12
13 ' Button invokes square-root calculation
14 Friend WithEvents cmdSquareRoot As Button
15
16 ' TextBox receives user's Integer input
17 Friend WithEvents txtInput As TextBox
18
19 ' Visual Studio .NET generated code
20
21 ' computes square root of parameter; throws
22 ' NegativeNumberException if parameter is negative
23 Public Function SquareRoot(ByVal value As Double) As Double
24
25 ' if negative operand, throw NegativeNumberException
26 If value < 0 Then
27 Throw New NegativeNumberException( _
28 "Square root of negative number not permitted")
29
30 End If
31
Fig. 11.5 FrmSquareRoot class throws an exception if an error occurs when
calculating the square root (part 1 of 2).