Visual Basic.NET How to Program (PDF)

Chapter 17 Files and Streams 785
58 ' show error if user specified invalid file
59 If (fileName = "" OrElse fileName = Nothing) Then
60 MessageBox.Show("Invalid File Name", "Error", _
61 MessageBoxButtons.OK, MessageBoxIcon.Error)
62
63 ' else enable all GUI buttons, except for Open File button
64 Else
65 cmdOpen.Enabled = False
66 cmdCredit.Enabled = True
67 cmdDebit.Enabled = True
68 cmdZero.Enabled = True
69 End If
70
71 End Sub ' cmdOpen_Click
72
73 ' invoked when user clicks Credit Balances, Debit Balances
74 ' or Zero Balances button
75 Protected Sub cmdGet_Click(ByVal senderObject As Object, _
76 ByVal e As System.EventArgs) Handles cmdCredit.Click, _
77 cmdZero.Click, cmdDebit.Click
78
79 ' convert senderObject explicitly to object of type Button
80 Dim senderButton As Button = CType(senderObject, Button)
81
82 ' get text from clicked Button, which stores account type
83 Dim accountType As String = senderButton.Text
84
85 ' used to store each record read from file
86 Dim record As CRecord
87
88 ' read and display file information
89 Try
90
91 ' close file from previous operation
92 If (input Is Nothing) = False Then
93 input.Close()
94 End If
95
96 ' create FileStream to obtain read access to file
97 input = New FileStream(fileName, FileMode.Open, _
98 FileAccess.Read)
99
100 txtDisplay.Text = "The accounts are:" & vbCrLf
101
102 ' traverse file until end of file
103 While True
104
105 ' get next CRecord available in file
106 record = CType(reader.Deserialize(input), CRecord)
107
108 ' store record's last field in balance
109 Dim balance As Double = record.Balance
Fig. 17.12 FrmCreditInquiry class is a program that displays credit inquiries
(part 3 of 7).