Three of the controls on the ToolBox let you access the computer's file system.
They are DriveListBox, DirListBox and FileListBox controls (see below figure)
, which are the basic blocks for building dialog boxes that display the host computer's
file system. Using these controls, user can traverse the host computer's file
system, locate any folder or files on any hard disk, even on network drives. The
files are controls are independent of one another, and each can exist on it's
own, but they are rarely used separately. The files controls are described next.
In a nutshell, the DriveListBox control is a combobox-like control that's automatically
filled with your drive's letters and volume labels. The DirListBox is a special
list box that displays a directory tree. The FileListBox control is a special-purpose
ListBox control that displays all the files in a given directory, optionally filtering
them based on their names, extensions, and attributes.
These controls often work together on the same form; when the user selects
a drive in a DriveListBox, the DirListBox control is updated to show the directory
tree on that drive. When the user selects a path in the DirListBox control, the
FileListBox control is filled with the list of files in that directory. These
actions don't happen automatically, however—you must write code to get the
job done.
After you place a DriveListBox and a DirListBox control on a form's surface,
you usually don't have to set any of their properties; in fact, these controls
don't expose any special property, not in the Properties window at least. The
FileListBox control, on the other hand, exposes one property that you can set
at design time—the Pattern property. This property indicates which files
are to be shown in the list area: Its default value is *.* (all files), but you
can enter whatever specification you need, and you can also enter multiple specifications
using the semicolon as a separator. You can also set this property at run time,
as in the following line of code:
File1.Pattern = "*.txt;*.doc;*.rtf"
Following figure shows three files controls are used in the design
of Forms that let users explore the entire structure of their hard disks.
The three File controls are not tied to one another. If you place all three
of them on a Form, you will see the names of all the folders under the current
folder, and so on. Each time you select a folder in the DirlistBox by double clicking
its name, its sub folders are displayed. Similarly , the FileListBox control will
display the names of all files in the current folder. Selecting a drive in the
DriveListBox control, however this doesn't affect the contents of the DirListBox.
To connect to the File controls, you must assign the appropriate values to
the properties. To compel the DirListBox to display the folders of the selected
drive in the DriveListBox, you must make sure that each time the user selects
another drive, the Path property of the DirListBox control matches the Drive property
of the DriveListBox.
After these preliminary steps, you're ready to set in motion the chain of events.
When the user selects a new drive in the DriveListBox control, it fires a Change
event and returns the drive letter (and volume label) in its Drive property. You
trap this event and set the DirListBox control's Path property to point to the
root directory of the selected drive:
Private Sub Drive1_Change()
' The Drive property also returns the volume label, so trim it.
Dir1.Path = Left$(Drive1.Drive, 1) & ":\"
End Sub
When the user double-clicks on a directory name, the DirListBox control raises
a Change event; you trap this event to set the FileListBox's Path property accordingly:
Private Sub Dir1_Change()
File1.Path = Dir1.Path
End Sub
Finally, when the user clicks on a file in the FileListBox control, a Click
event is fired (as if it were a regular ListBox control), and you can query its
Filename property to learn which file has been selected. Note how you build the
complete path:
Filename = File1.Path
If Right$(Filename, 1) <> "\" Then Filename = Filename & "\"
Filename = Filename & File1.Filename
The DirListBox and FileListBox controls support most of the properties typical
of the control they derive from—the ListBox control—including the
ListCount and the ListIndex properties and the Scroll event. The FileListBox control
supports multiple selection; hence you can set its MultiSelect property in the
Properties window and query the SelCount and Selected properties at run time.
The FileListBox control also exposes a few custom Boolean properties, Normal,
Archive, Hidden, ReadOnly, and System, which permit you to decide whether files
with these attributes should be listed. (By default, the control doesn't display
hidden and system files.) This control also supports a couple of custom events,
PathChange and PatternChange, that fire when the corresponding property is changed
through code. In most cases, you don't have to worry about them, and I won't provide
examples of their usage.
The problem with the DriveListBox, DirListBox and FileListBox controls is that
they're somewhat outdated and aren't used by most commercial applications any
longer. Moreover, these controls are known to work incorrectly when listing files
on network servers and sometimes even on local disk drives, especially when long
file and directory names are used. For this reason, I discourage you from using
them and suggest instead that you use the Common Dialog controls for your FileOpen
and FileSave dialog boxes. But if you need to ask the user for the name of a directory
rather than a file, you're out of luck because—while Windows does include
such a system dialog box, named BrowseForFolders dialog—Visual Basic still
doesn't offer a way to display it (unless you do some advanced API programming).
Fortunately, Visual Basic 6 comes with a new control—the ImageCombo control—that
lets you simulate the appearance of the DriveListBox control. It also offers you
a powerful library—the FileSystemObject library—that completely frees
you from using these three controls, if only as hidden controls that you use just
for quickly retrieving information on the file system.
See Also
