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Creating and Using Files

Naming a File

A file must have a name, so that it can be retrieved from the location where it is stored. That location can be the internal hard drive, in which case the folder containing the file must be specified, or an external storage device such as a floppy disk, Zip drive, or memory stick. It is customary (but optional) for the name to include a program-specific extension, three letters after a dot, that denotes the file's purpose. WORD saves documents with a .doc extension, EXCEL saves sheets with a .wks extension.

The file name can be built into the program, or may be provided by the user during program execution. The file name is a string, and includes the location information (e.g., a:\myfile.dat).

Eliciting the File Name from the User

The simplest way to ask the user to provide a file name is with a label and text box, wherein the user simply types the name (including the location information). It is far more elegant and user-friendly, though, for the program to get the file name in a dialog box. That allows the user to locate the file with visual cues, and also to avoid input errors that might result in an illegal file name.

Using the File Dialog Box

There are two things we might do with the file dialog box, Open and Save. The same dialog box control is used for both operations, with the programmer setting a parameter to determine which sort of box appears. The dialog box control is not part of the VB6 default set of controls, and so must be added to the toolbox for the current program. This is accomplished by clicking on the Project Menu, then Components..., then the Controls tab. Add the Microsoft Common Dialog Control 6.0 to the toolbox by checking the box to the left, then clicking on OK. After you save the project, the control will always appear in the toolbox for that project. Drag the control onto the form; it is never visible, so it doesn't matter how big it is or where you place it.

Coding the file dialog box can be fairly tricky, as there are a lot of options. However, many of them don't matter too much, and what I show here is sufficient. 

The code below might go in the subroutine associated with the Open ...  item in the File Menu. It is designed to read a set of N scores, to be referred to within the program as the array X() from a file with a .dat extension..

On Error GoTo Errorcheck  'Sets up error
  'trapping, to allow cancellation

Dim Numfile as Long

Dim I as Integer

Dim X(N) as Single

Dim Infilename as String

CommonDialog1.CancelError = True  'Allows
  'Cancel Button to operate

CommonDialog1.DialogTitle = "Open File"

CommonDialog1.Filter = "Data Files |
  *.dat|All Files | *.*"  'Optional
  'statement that filters the files so
  'that only those with particular
  'extensions that will be displayed

CommonDialog1.Flags = OFN_FILEMUSTEXIST Or
  OFN_HIDEREADONLY  'Optional
  'statement that limits the files
  'that will be displayed

CommonDialog1.Action = 1  'Opens the 
  'dialog box with an Open File setting

Reading from a File

Numfile = FreeFile  'Freefile is an
  'internal VB variable

Infilename = CommonDialog1.Filename

Open Infilename For Input As #Numfile Len
   = 1024

For I = 1 to N  

   Input #Numfile, X(I) 'reads an array
      'that will be called X 

Next I

Input #Numfile, Username 'reads a value
   'that will be stored as Username

Close Numfile
' other statements that check the data 'might go here

Exit sub

Errorcheck:  'Error trapping statements

If Err = 32755 then Exit Sub  'Checking if
 'Cancel Button was clicked

End Sub

Saving to a File

Information is saved by "printing" it to a file. This code might go in the subroutine associated with the Save As... item in the File Menu. Here, we are saving a set of M scores that had previously been stored publicly in the array Y() into a file with a .dat extension..

Dim Numfile As Long

Dim I As Integer

Dim Outfilename as String

On Error GoTo Saverror 'sets up error
  'trapping

CommonDialog1.CancelError = True

CommonDialog1.DialogTitle = "Save File
   (As)"

CommonDialog1.Filter = "Data Files
   (*.dat)| *.dat"

CommonDialog1.Flags = OFN_OVERWRITEPROMPT
   Or OFN_PATHMUSTEXIST Or
   OFN_HIDEREADONLY

CommonDialog1.Action = 2 'Opens the 
  'dialog box with a Save File setting

Outfilename = CommonDialog1.Filename

Numfile = FreeFile

Open Outfilename For Output As #Numfile

For I = 1 To M  'prints an array of data
 'called Y

   Print #Numfile, Y(I)

Next I

Print #Numfile, Username 'prints a stored
 'variable

Print #Numfile, Userage  'prints another
 'stored variable

Close Numfile

Exit sub

Saverror:  'Error trapping statements

If Err = 32755 then Exit Sub  'Checking if
 'Cancel Button was clicked

End Sub