RowSourceType Property (User-Defined Function) - Code Argument Values

Microsoft Access Visual Basic

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Function functionname (fld As Control, id As Variant, row As Variant, col As Variant, code As Variant) As Variant

The Function procedure has the following five required arguments.

Argument	Description
fld	A control variable that refers to the list box or combo box being filled.
id	A unique value that identifies the control being filled. This is useful when you want to use the same user-defined function for more than one list box or combo box and must distinguish between them. (The example sets this variable to the value of the Timer function.)
row	The row being filled (zero-based).
col	The column being filled (zero-based).
code	An intrinsic constant that specifies the kind of information being requested.

Note Because Microsoft Access calls a user-defined function several times to insert items into a list, often you must preserve information from call to call. The best way to do this is to use Static variables.

Microsoft Access calls the user-defined function by repeatedly using different values in the code argument to specify the information it needs. The code argument can use the following intrinsic constants.

Constant	Meaning	Function returns
acLBInitialize	Initialize	Nonzero if the function can fill the list; False (0) or Null otherwise.
acLBOpen	Open	Nonzero ID value if the function can fill the list; False or Null otherwise.
acLBGetRowCount	Number of rows	Number of rows in the list (can be zero); –1 if unknown.
acLBGetColumnCount	Number of columns	Number of columns in the list (can't be zero); must match the property sheet value.
acLBGetColumnWidth	Column width	Width (in twips) of the column specified by the col argument; –1 to use the default width.
acLBGetValue	List entry	List entry to be displayed in the row and column specified by the row and col arguments.
acLBGetFormat	Format string	Format string to be used to format the list entry displayed in the row and column specified by the row and col arguments; –1 to use the default format.
acLBEnd	End (the last call to a user-defined function always uses this value)	Nothing.
acLBClose	(Not used)	Not used.

Microsoft Access calls your user-defined function once for acLBInitialize, acLBOpen, acLBGetRowCount, and acLBGetColumnCount. It initializes the user-defined function, opens the query, and determines the number of rows and columns.

Microsoft Access calls your user-defined function twice for acLBGetColumnWidth— once to determine the total width of the list box or combo box and a second time to set the column width.

The number of times your user-defined function is called for acLBGetValue and acLBGetFormat to get list entries and to format strings varies depending on the number of entries, the user's scrolling, and other factors.

Microsoft Access calls the user-defined function for acLBEnd when the form is closed or each time the list box or combo box is queried.

Whenever a particular value (such as the number of columns) is required, returning Null or any invalid value causes Microsoft Access to stop calling the user-defined function with that code.

Show Tip

You can use the Select Case code structure from the example as a template for your own RowSourceType property user-defined functions.

Example

The following user-defined function returns a list of the next four Mondays following today's date. To call this function from a list box control, enter ListMondays as the RowSourceType property setting and leave the RowSource property setting blank.

Function ListMondays(fld As Control,id As Variant, _
    row As Variant,col As Variant,code As Variant) _
     As Variant
    Dim intOffset As Integer
    Select Case code
        Case acLBInitialize            ' Initialize.
            ListMondays = True
        Case acLBOpen                    ' Open.
            ListMondays = Timer        ' Unique ID.
        Case acLBGetRowCount            ' Get rows.
            ListMondays = 4
        Case acLBGetColumnCount    ' Get columns.
            ListMondays = 1
        Case acLBGetColumnWidth    ' Get column width.
            ListMondays = -1            ' Use default width.
        Case acLBGetValue                ' Get the data.
            intOffset = Abs((9 - Weekday(Now))Mod 7)
            ListMondays = Format(Now() + _
             intOffset + 7 * row,"mmmm d")
    End Select
End Function

The next example uses a static array to store the names of the databases in the current directory. To call this function, enter ListMDBs as the RowSourceType property setting and leave the RowSource property setting blank.

Function ListMDBs(fld As Control, id As Variant, _
     row As Variant, col As Variant, _
     code As Variant) As Variant
    Static dbs(127) As String, Entries As Integer
    Dim ReturnVal As Variant
    ReturnVal = Null
    Select Case code
        Case acLBInitialize                ' Initialize.
            Entries = 0
            dbs(Entries ) = Dir("*.MDB")
            Do Until dbs(Entries) = "" Or Entries >= 127
                Entries = Entries+1
                dbs(Entries) = Dir
            Loop
            ReturnVal = Entries
        Case acLBOpen                        ' Open.
            ' Generate unique ID for control.
            ReturnVal = Timer
        Case acLBGetRowCount            ' Get number of rows.
            ReturnVal = Entries
        Case acLBGetColumnCount    ' Get number of columns.
            ReturnVal = 1
        Case acLBGetColumnWidth    ' Column width.
            ' -1 forces use of default width.
            ReturnVal = -1
        Case acLBGetValue                    ' Get data.
            ReturnVal = dbs(row)
        Case acLBEnd                        ' End.
            Erase dbs
    End Select
    ListMDBs = ReturnVal
End Function

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