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]
|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.
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.
|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.
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
To check whether STUDENTS.DAT file is existing or not:
If dir(“students.dat”) = “” then
MsgBox “File students.dat is missing”
Process the file
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.
|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.
|Line Input #||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.
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.
|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.
|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.
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.
- Start a new project using File-> New Project and select Standard Exe as the project type.
- Place FileListBox, DirListBox and DriveListBox on the left of the form (as shown in figure 10.1).
- Place three label controls to display messages File, Directories and Drives at the top of each of the controls created in the previous step.
- Place a Frame control on the right of the form.
- Place a collection of label controls. These label controls are used to display the information regarding file.
- 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.
- Place a command button at the bottom. This is used to quit the application.
- 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.
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.
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
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
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 & “\”
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
Listing 10.3: Code for click event of File control.
Public Sub GetFileAttributes(attr As Integer, lstbox As ListBox)
‘clear all items from listbox
‘check for read-only attribute
If (attr And vbReadOnly) <> 0 Then
lstbox.AddItem “Read Only”
‘ check for Hidden attribute
If (attr And vbHidden) <> 0 Then
‘check for System attribute
If (attr And vbSystem) <> 0 Then
‘check for Archive attribute
If (attr And vbArchive) <> 0 Then
Listing 10.4: Code for GetFileAttributes user-defined procedure
Also write code for Quit button, which contains a single statement which is Unload Me.
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.
Figure 10.2:Details of file IO.SYS
If you select root directory and the select IO.SYS, the display should be identical to figure 10.2.
You can enhance the application by adding a button to display the content of the selected file. However, for this you need to take only text files (files that are readable) into account.