Visual Basic.NET How to Program (PDF)

Chapter 17 Files and Streams 811
151
152 Return False ' failure
153 End Try
154
155 Return True ' success
156 End Sub ' AddRecord
157
158 End Class ' CTransaction
Fig. 17.18 CTransaction class handles record transactions for the transactionprocessor
case study (part 4 of 4).
Class CTransaction contains methods OpenFile, GetRecord and Add-
Record. Method OpenFile (lines 26–75) uses Const FileMode.OpenOrCreate
(line 32) to create a FileStream object from either an existing file or one not yet created.
Lines 35–38 use this FileStream to create BinaryReader and BinaryWriter
objects for reading and writing bytes to the file. If the file is new, lines 54–64 populate the
FileStream object with empty records. Students might recall that we used these techniques
in Section 17.8.
Method GetRecord (lines 78–128) returns the record associated with the accountnumber
parameter. Lines 85–86 instantiate a CRandomAccessRecord object that will
store the file data. If the account parameter is valid, lines 103–104 call method Seek of the
FileStream object, which uses the parameter to determine the position of the specified
record in the file. Lines 107–110 then call methods ReadInt32, ReadString and
ReadDouble of the BinaryReader object to store the file data in the CRandomAccessRecord
object. Line 113 returns the CRandomAccessRecord object. We used
these techniques in Section 17.10.
Method AddRecord (lines 131–156) inserts a record into the file. Lines 138–139 call
method Seek of the FileStream object, which uses the account-number parameter to
locate the position which to insert the record in the file. Lines 142–145 call the overloaded
Write methods of the BinaryWriter object to write the CRandomAccessRecord
object’s data to the file. We used these techniques in Section 17.9. Note that, if an error
occurs when adding the record (i.e., either the FileStream or the BinaryWriter
throws an IOException), lines 149–152 notify the user of the error and return False
(failure).
Transaction-Processor GUI
The GUI for this program consists of a window containing internal frames (an MDI). Class
FrmTransactionProcessor (Fig. 17.19) is the parent window, which acts as the
driver for the application and displays one of its children windows—an object of type
FrmStartDialog (Fig. 17.20), FrmNewDialog (Fig. 17.21), FrmUpdateDialog
(Fig. 17.22) or FrmDeleteDialog (Fig. 17.23). FrmStartDialog allows the user to
open a file containing account information and provides access to the FrmNewDialog,
FrmUpdateDialog and FrmDeleteDialog internal frames. These frames allow us-