1. File Handling

2. Data Files
Programs that can only get data from the user interface are limited.
data must be entered each time.
only small amounts of data can be processed.
Both limitations can be circumvented by storing data in text files.

3. Data Files A text file is one that contains only ASCII characters.
Operations that can be performed on files:
Read
Write
Append

4. Microsoft Common Dialog Controls
Files as Objects
Dealing with files will require specifying their names and paths. To facilitate this the standard Windows Dialog boxes can be used.
But VB6 doesn’t automatically have access to these, so before we can deal with files we need to add some functionality.
We need to add the
Microsoft Common Dialog Controls

5. Adding Components
Click on Project
Components…

6. Adding Controls Scroll down to Microsoft Common Dialog Control 6.0
Check it
Click OK

7. Including Dialog Controls
Observe that the icon representing the control for creating the standard Microsoft Windows dialog windows has been added to the toolbox.

8. Including Dialog Contols
Like any other control, The Common Dialog contol must be added to the interface for a project to use it.
Simply click the icon in the toolbox And click on the form.
The control cannot be resized because it does not appear in the interface.

9. Common Dialog Control
The Common Dialog Control allows VB6 projects to use a set of dialog boxes that are common in Windows applications.
The methods in the Common Dialog Control include routines to perform various tasks:
Change a font
Change colours
Open a file
Print
Provide help
Save a file

10. Dialog Control Properties
Like all controls, the Common Dialog has properties:
Filter –specifies file types shown
InitDir – specifies the initial directory
CancelError – determines how clicking “Cancel” will be treated
Flags – sets various conditions for the dialog

11. Dialog Control Properties
For a Dialog Control called Test, a program could set properties at runtime like these…
cdlTest.Filter = "Text files|*.txt"
cdlTest.Flags = cdlOFNFileMustExist
cdlTest.InitDir = "C:\My Music"

12. Common Dialog Control
The Dialog Control provides access to the common dialogs, but using these dialogs requires further tools.
To read a text file, for example, new classes are needed:
FileSystemObject
TextStream
These classes have properties and methods.

13. TextStream Methods Close Read ReadAll ReadLine Skip SkipLine Write
WriteBlankLines
WriteLine

14. Incorporating New References
Adding a new collection of classes simply makes them available.
We also need to create References to them.

15. Incorporating New References
Click on Project,
then References…
This dialog appears.

16. Incorporating New References
Scroll down and Select
Microsoft Scripting Runtime

17. Dealing with Files
This gives us the tools, now we need to write the code to use them.

18. Reading a File To read data from a file:
create a reference to an object of the FileSystemObject class
Dim aFile As FileSystemObject
create a reference to an object of the TextStream class
Dim FileContent As TextStream

19. Reading a File use the common dialog control to obtain the file name
cdlTest.ShowOpen
create a new FileSystemObject object
Set aFile = New FileSystemObject
use the OpenTextFile method on the FileSystemObject to obtain a TextStream object
Set FileContent = aFile.OpenTextFile(cdlTest.FileName)

20. Reading a File
use the ReadAll method on the TextStream object to obtain the file contents as a string
txtFileContent.Text = FileContent.ReadAll
close the file
FileContent.Close

21. Reading a File Private Sub cmdOpenFile_Click()
Dim aFile As FileSystemObject
Dim FileContent As TextStream '
cdlTest.Filter = "Text files (*.txt)|*.txt|All files (*.*)|*.*"
cdlTest.Flags = cdlOFNFileMustExist
cdlTest.ShowOpen
Set aFile = New FileSystemObject
Set FileContent = aFile.OpenTextFile(cdlTest.FileName)
txtFileContent.Text = FileContent.ReadAll
FileContent.Close
Exit Sub

22. Error Handling
It’s always best to handle errors with program code rather than system messages (and aborted programs).
VB6 provides an easy way to deal with errors.
Private Sub ReadAFile(…)
‘branch to error handler if error occurs
OnError GoTo ErrorHandler
‘regular subroutine
Exit Sub
‘write error handling code
ErrorHandler:
‘code to deal with errors
End Sub

23. A Complete Example Private Sub cmdOpenFile_Click()
On Error GoTo ErrorHandler
Dim aFile As FileSystemObject
Dim FileContent As TextStream
cdlTest.ShowOpen
Set aFile = New FileSystemObject
Set FileContent = aFile.OpenTextFile(cdlTest.FileName)
txtFileContent.Text = FileContent.ReadAll
FileContent.Close
Exit Sub
ErrorHandler:
'user pressed Cancel - do nothing
Dim messg As String
messg = Str(Err.Number) & ": " & Err.Description
MsgBox (messg)
End Sub