Recipe 7.8 Fill a List Box with a List of Files

7.8.1 Problem

You need to present your users with a sorted list of files with a specific filename extension in a particular directory. You found the Dir function, but you can't find a way to get this information into a list box. Is there a way to do this?

7.8.2 Solution

This problem provides the perfect opportunity to use the past three solutions. It involves creating a list-filling callback function, passing arrays as parameters, and sorting an array. In addition, you'll fill that array with a list of files matching a particular criterion, using the Dir function.

Load the form frmTestFillDirList from 07-08.MDB. Enter a file specification into the text box (for example, c:\*.exe). Once you leave the text box (by pressing either Tab or Return), the code attached to the AfterUpdate event will force the list box to requery. When that happens, the list box will fill in with the matching filenames. Figure 7-12 shows the results of a search for c:\\*.*.

Figure 7-12. frmTestFillDirList, searching for *.* in the C:\ folder

To include this functionality in your own applications, follow these steps:

Table 7-6. Property settings for the controls on the directory list form

Control	Property	Setting
Text box	Name	txtFileSpec
	AfterUpdate	[Event Procedure]
List box	Name	lstDirList
	RowSourceType	FillList
	AfterUpdate	[Event Procedure]

Rather than resizing the array for each matching file name, the FillDirList function traps the error that occurs when the array is full, and resizes it 100 slots at a time. Using the Redim Preserve statement is quite expensive in VBA, and you should consider looking for ways to call it as seldom as possible. In this example, the code resizes the array to the correct size once it's done filling in all the file names.

7.8.3 Discussion

The list box in this example uses a list-filling callback function, FillList, to supply its data. (See the Solution in Recipe 7.5 for information on callback functions.) Here's the code:

Private Function FillList(ctl As Control, _
 varID As Variant, lngRow As Long, lngCol As Long, _
 intCode As Integer)
    Static astrFiles( ) As String
    Static intFileCount As Integer

    Select Case intCode
        Case acLBInitialize
            If Not IsNull(Me.txtFileSpec) Then
                intFileCount = FillDirList(Me.txtFileSpec, astrFiles( ))
            End If
            FillList = True

        Case acLBOpen
            FillList = Timer

        Case acLBGetRowCount
            FillList = intFileCount

        Case acLBGetValue
            FillList = astrFiles(lngRow)

        Case acLBEnd
            Erase astrFiles
    End Select
End Function

In FillList's acLBInitialize case, it calls the FillDirList function to fill in the astrFiles array, based on the value in the txtFileSpec text box. FillDirList fills in the array, calling acbSortArray along the way to sort the list of files, and returns the number of files it found. Given that completed array, FillList can return the value from the array that it needs when requested in the acLBGetValue case. It uses the return value from FillDirList, the number of files found, in response to the acLBGetRowCount case.

There's also an interesting situation you should note in the FillList and FillDirList routines. FillList declares a dynamic array, astrFiles, but doesn't give a size because it doesn't yet know the number of files that will be found. FillList passes the array off to FillDirList, which adds filenames to the array based on the file specification until it doesn't find any more matches. FillDirList returns the number of matching filenames, but it also has the side effect of having set the array's size and filled it in. Here's the code that does the work. This code fragment uses the ReDim Preserve keywords to resize the array every time it finds a matching filename:

' Set the filespec for the dir( ) and get the first filename.
strTemp = Dir(strFileSpec)
Do While Len(strTemp) > 0
    intNumFiles = intNumFiles + 1
    astrFiles(intNumFiles - 1) = strTemp
    strTemp = Dir
Loop

FillDirList uses the Dir function to create the list of files. This function is unusual in that you call it multiple times. The first time you call it, you send it the file specification you're trying to match, and Dir returns the first matching filename. If it returns a nonempty value, you continue to call it, with no parameters, until it does return an empty value. Each time you call Dir, it returns the next matching filename.

Once FillDirList has finished retrieving the list of filenames, it sorts the names in the array. Its return value is the number of files it found. The following code shows how this works:

If intNumFiles > 0 Then
    ReDim Preserve astrFiles(intNumFiles - 1)
    acbSortArray astrFiles( )
End If
FillDirList = intNumFiles

Note that when Access calls the list-filling callback function, values for the lngRow and lngCol parameters are always zero-based. Therefore, when you use arrays within callback functions, you should always consider using zero-based arrays to hold the data you'll display in the control. If you don't, you'll always be dealing with "off by one" errors. Using a zero-based array will mean that the row values (sent to your code in lngRow) will match your array indices.