File handling

A file is a collection of bytes stored on the disk with a given name (called as filename). Every development tool provides access to these files on the disk.  In this chapter we will understand how to access and manipulate files using Visual Basic.

There are three special controls, called as File controls, which deal with files and directories. We will also understand how to use these controls in this chapter.

File handling

The following are three important steps in handling a file.

¨         Opening the file

¨         Processing the file, i.e. either reading the content of the file or writing the required data into file or both.

¨         Closing the file

File access types

Depending upon the requirement you can use any of the three different file access types:

Sequential	For reading and writing text files in continuous blocks.
Random	For reading and writing text or binary files structured as fixed-length records.
Binary	For reading and writing arbitrarily structured files.

Opening the file using Open statement

A file is opened using OPEN statement in Visual Basic. At the time of opening a file, you have to specify the following.

¨         Name of the file to be opened

¨         The mode in which file is to be opened. The mode specifies which operations are allowed on the file.

¨         File number. Each open file contains a file number.  This file number is used to access the file once the file is opened.  The file number must be unique.

Open pathname For mode [Access access] [lock] As [#] filenumber [Len=reclength]

Option	Meaning
Pathname	Name of the file to be opened.
Mode	Specifies the mode in which file is to be opened. Valid modes: Append, Input, Output, Binary and Random. If unspecified then Random is taken.
Access	Specifies the operations that are permitted on the open file.  Valid values: Read, Write, or ReadWrite.
Lock	Specifies the operations restricted on the file opened by other users. Value values: Shared, Lock Read, Lock Write, and Lock Read Write.
Filenumber	A number in the range 1 to 511. This number must be unique among open files. Use FreeFile function to obtain the next available number.
RecLength	Specifies the size of each record in random files.  It should be <= 32767. For sequential files, this is the number of characters buffered. This is ignored, if mode is Binary.

If the file is not existing then a new file with the given name is created in Append, Binary, Output and Random modes.

Examples:

To open TEST.TXT file in input mode:

Open “TEST.TXT” for input as #1

To open NUMBER.DAT file in Binary mode:

Open “NUMBER.DAT” for binary access write as #1

To open STUDENTS.DAT in random mode with a record length of 10:

Open  “STUDENTS.DAT” for random as #1 len = 10

To get next available file number and then use it:

'FreeFile function returns the number that can be used as the file 'number while opening the file

Fn = FreeFile

Open “TEST.TXT” for input as #fn

Functions related to files

The following are the functions that are used with files.

Function	Meaning
Dir	Returns the name of the file that matches the given name. If file is not existing then it returns "" (null string).
FileLen	Returns the length of the file in bytes.
LOF	Returns the length of an open file in bytes.
EOF	Returns true, if the specified file has reached end-of-file marker.
FreeFile	Returns the next available file number.
Seek	Sets or returns the position at which file pointer is currently positioned. For random files it returns the number of records read or written so far.
Filecopy	Copies the given source file to target file.
GetAttr	Returns the attributes of the given path.
SetAttr	Changes the attributes of the specified file to the given attributes.
FileDateTime	Returns the date and time when file was last modified or created.
Loc	Returns the current position of file pointer of an open file.

Table 10.1: Functions related to file handling.

Example:

To find out the length of file CHARS.TXT:

fl = FileLen("c:\vb60\chars.txt")

 

To find out whether file with the file number 1 has reached end-of-file:

if EOF(1) then

     …

end if

To check whether STUDENTS.DAT file is existing or not:

If  dir(“students.dat”)  = “” then

   MsgBox  “File students.dat is missing”

Else

   Process the file

End if

Statement related to file Input and output

The following statements are used to perform input or output to file and other operations such as opening and closing.

Statement	Meaning
Close	Closes an open file
Get	Read a record from the given position of the specified file.
Input( )	Returns the specified number of characters from the given file.
Input #	Reads data into specified list of variables from the given file.
Line Input #	Reads a complete line from the given file.
Open	Opens the given file in the specified mode.
Print #	Prints the specified data to the given file.
Put	Writes a record to the given position of the specified file.
Write #	Writes the specified data to the given file.

Table 10.2: Statements related to file handling.

Not all statements are available in all modes. So, the following table shows the availability of each statement in each of the three access types.

In the table X denotes the availability of the command in the mode.

Statement	Sequential	Random	Binary
Close	X	X	X
Get		X	X
Input( )	X		X
Input #	X
Line Input #	X
Open	X	X	X
Print #	X
Put		X	X
Write #	X		`

Table 10.3: The availability of the statements in three access modes.

Note: See chapter 12, to understand how to use statements related to sequential files. Detail discussion of Random and Binary files is beyond the scope of this material. Please see on-line documentation if you are interested.

File Controls

Visual Basic provides a set of controls, which are used to display files, directories and drives. These controls are part of standard controls. The following are the file controls and what they do.

Control	Meaning
FileListBox	Displays a list of files. This is a listbox that displays list of files from the path specified using Path property.
DirListBox	Displays hierarchical list of directories.
DriveListBox	Displays the list of drives available in the system.

Table 10.4: File controls.

Properties and Events of FileListBox

The following are specific events and properties of FileListBox.

Type	Name	Meaning
Property	Filename	Contains the name of the file selected by user.
	Path	Contains the name of the directory from where list of files is taken.
	Pattern	Specifies the pattern that is used to filter filenames, such as *.exe.
	System	Specifies whether system files are to be displayed or not.
	Readonly	Specifies whether readonly files are to be displayed or not.
	Hidden	Specifies whether hidden files are to be displayed or not.
	Archive	Specifies whether archive files are to be displayed or not.
Event	Pathchange	Occurs whenever path is changed.
	Patternchange	Occurs whenever pattern is changed.

Table 10.5: Properties and events that are specific to FileListBox

Note: FileListBox also has properties that are available for listbox, such as MultiSelect, ListCount etc., but it doesn’t have the methods that are found with listbox, such as Additem.

Sample Application

Let us develop an application that used file controls and file-related functions. This application allows user to select a file from any directory and drive using three file-related controls mentioned above. Once user selects a file, details of the selected file, such as length, attributes and date of last modification will be displayed. 

 

Figure 10.1: Sample application at design time.

The following are the steps required to create the sample application.

1.      Start a new project using File-> New Project and select Standard Exe as the project type.

2.      Place FileListBox, DirListBox and DriveListBox on the left of the form (as shown in figure 10.1).

3.      Place three label controls to display messages File, Directories and Drives at the top of each of the controls created in the previous step.

4.      Place a Frame control on the right of the form.

5.      Place a collection of label controls. These label controls are used to display the information regarding file.

6.      Also place a Listbox. This is used to display the attributes of the file.  A listbox is selected because a file may contain more than one attribute, such as Readonly, Hidden etc.

7.      Place a command button at the bottom. This is used to quit the application.

8.      Arrange all controls as shown in figure10.1.

Note:  A frame is used only to group controls physically to show that all the controls are related.

Changing properties

After having placed required controls on the form, now let us concentrate on changing required properties to get required appearance and functionality.

First let us concentrate on the properties of file controls.  For Directory list box and Drive list box no property needs to be changed. But for File list box some properties are to be changed in order to display all types of files. By default only files of type Archive and Read-only only will be displayed. But as we want to display all types of files, we have to set Hidden and System properties to True. Remember by default Archive and ReadOnly properties are already set to True.

The following table lists out the properties to be changed for all other controls on the form.

Control	Property	Value
Frame1	Caption	Details
	Fontbold	True
Label1	Caption	Files
Label2	Caption	Directories
Label3	Caption	Drives
Label4	Caption	Filename
Label5	Name	Lblname
	Caption	“”
	Borderstyle	1-Fixed Single
Label6	Caption	Size
Label7	Name	Lblsize
	Caption	“”
	Borderstyle	1-Fixed Single
Label8	Caption	Attributes
Label9	Caption	Last Updated
Label10	Name	Lbldate
	Caption	“”
	Borderstyle	1-Fixed Single
List1	Name	Lstattributes
Command1	Name	Cmdquit
	Caption	&Quit

Note: The default names of the label control may not exactly match the names of your label controls, if the order in which you created the label control is different. Use figure 10.1, as the guide and change the properties.

Writing Code to connect file controls

At this stage, if you run the project and select a different directory, it doesn’t change the list of files. But it should. In the same way if you select a different drive, it doesn’t change the list of directories. But again it should. So, we first concentrate on connecting these three controls. That means when user changes the drive, then automatically directory list box should display the list of directories from new drive. The same is true with selecting a different directory in directory list box (it should change list of files in file list box).

Whenever user changes the selection of drive, Change event occurs for Drive list box. Take the drive selected by user using Drive property of DriveListBox and set it to Path property of DirListBox. Then DirectoryListBox will get the list of directories from new drive. Here is the code to do that.

Private Sub Drive1_Change()

  Dir1.Path = Drive1.Drive

 

End Sub

Listing 10.1: Code to connect drive list box with directory listbox.

Whenever user select a different directory,  Change event for DirListBox occurs. Take new path and assign that value to Path property of FileListBox to get new list of file from newly selected directory. Here is the code to do that.

Private Sub Dir1_Change()

    File1.Path = Dir1.Path

End Sub

Listing 10.2: Code to connect DirListBox to FileListBox.

Now let us get to crux. Whenever user selects a file in FileListBox, we have to display information regarding the selected file. The information is, Full name of the file, Size of the file, Attributes of the file and Date and time on which the file was last updated.

Getting size and date of last updation is quite simple. It is just a matter of using FileLen and FileDateTime functions.

Getting full filename is also done except in one case. Path property of FileListBox contains the path in which file is existing. Filename property contains the name of the file. So to get complete filename we have to concatenate these two values. As path is to be separated from filename using a backslash (\), we also have to concatenate a backslash after path. However, when path is referring to root directory, backslash is already there in the path. Adding an extra backslash is going to make path invalid. So we have to see whether the last character is a backslash. It can be done by taking the right most character using Right function and comparing it with backslash. Only when the rightmost character is not a backslash we add backslash at the end of path. Listing 10.3, show the code to do this.

Attributes of a file can be obtained using GetAttr function. But this function returns an integer, which is to be decoded to get information regarding attributes. The value returned by GetAttr is to be bitwise Anded with predefined numbers to know the attributes of the file. For example, to know whether file is readonly or not we have to use bitwise And operator to perform bitwise Anded between the value returned by GetAttr and constant vbReadOnly.

We will write a procedure that takes the attributes integer and a listbox. The procedure determines the attributes of the file and adds items with attribute names to listbox. We name this procedure as GetFileAttributes.  Listing 10.4, show this procedure.

Private Sub File1_Click()

Dim fn As String

  ' form complete filename

  fn = File1.Path

  ‘ if rightmost character is not backslash then add a backslash

  If Right(File1.Path, 1) <> "\" Then

        fn = fn & "\"

  End If

 

  fn = fn & File1.FileName

 

  lblname.Caption = fn

 

  lblsize.Caption = FileLen(fn)

  lbldate.Caption = FileDateTime(fn)

 

  ‘ call procedure to populate listbox with attributes of the file

  GetFileAttributes GetAttr(fn), Lstattributes

End Sub

Listing 10.3: Code for click event of File control.

Public Sub GetFileAttributes(attr As Integer, lstbox As ListBox)

  ‘clear all items from listbox

  lstbox.Clear

 

  ‘check for read-only attribute

  If (attr And vbReadOnly) <> 0 Then

        lstbox.AddItem "Read Only"

  End If

  ‘ check for Hidden attribute

  If (attr And vbHidden) <> 0 Then

        lstbox.AddItem "Hidden"

  End If

 ‘check for System attribute

  If (attr And vbSystem) <> 0 Then

        lstbox.AddItem "System"

  End If

 ‘check for Archive attribute

  If (attr And vbArchive) <> 0 Then

        lstbox.AddItem "Archive"

  End If

 

End Sub

Listing  10.4: Code for GetFileAttributes user-defined procedure

Also write code for Quit button, which contains a single statement which is Unload Me.

Test Run

Now run the project by pressing F5. First you see list of files from current directory. Change directory by double clicking on a new directory in the Dirlist box. This should change the list of files. Also test whether a change in drive list box is changing the list of directories.