ACC: Bound Multi-Selection List Box Always Returns Null

• Microsoft Access versions 7.0, 97

SYMPTOMS

Moderate: Requires basic macro, coding, and interoperability skills.

A multiple-selection list box bound to a field in a form's underlying record source through the ControlSource property returns a Null value regardless of the selection or selections made in the list.

This article assumes that you are familiar with Visual Basic for Applications and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Visual Basic for Applications, please refer to your version of the "Building Applications with Microsoft Access" manual.

CAUSE

A bound control can only return and represent a single value stored in a field in the form's underlying recordset. Because multiple selections cannot be represented in a single field, a Null value is returned.

RESOLUTION

The Selected property or ItemsSelected collection can be used to retrieve information regarding which items are selected in a multiple-selection list box.

You will find information below that demonstrates one way to simulate binding a multiple-selection list box to a single field by storing and managing a semicolon-delimited list of values. For example, if your list box is filled with different fruits, one possible combination of selections might be stored as "Apples;Oranges;Bananas."

MORE INFORMATION

Drawbacks to Storing Multiple Selections in a Single Field

Although this article demonstrates how to store multiple selections from a multiple-selection list box into a single field, you should realize that it is not good database design, and you should explore alternatives. Storing a list of values in a single field violates basic rules of database normalization theory. First Normal Form in Normalization theory states that all column (field) values must be atomic, or indivisible. For example, in a single field you should not store a list of items such as "Apples, Oranges, Bananas" because there is no simple way to manipulate, sort, or query on these values independently of one another.

The following example demonstrates how to create a multiple-selection list box in the sample database Northwind.mdb's Customers form that enables you to select a list of "many" employees for each customer. Perhaps a customer should only work with a set of specific employees assigned by this list box. A new field called Employees is added to the Customers table to hold a semicolon-delimited list of Employee ID numbers managed by a multiple- selection list box to be added to the Customers form.

The disadvantage of this design is that you would not be able to determine what customers are currently assigned to a particular employee, or vice versa.

Ideally, because "many" selections need to be stored for the "one" record, you might want to explore storing these selections in a separate "many" table rather than in a single (or multiple fields) in the "one" table. The assignment of employees to customers is a many-to-many relationship such that "one" customer can have "many" employees and "one" employee can have "many" customers. Good database design dictates that you should create a third table, often called a "junction" or "assignment" table between the many-to-many tables. In the Northwind database, the Order Details table offers an excellent example of this. The Order Details table is positioned between the Orders and Products tables. Rather than assigning many products to one order using a multiple-selection list box, a subform is used in the Orders form to edit the Order Details junction table.

How to Simulate Binding a Multiple-Selection List Box to a Field

When a control is bound to a field in the underlying recordset of the form, Microsoft Access does two things automatically for you:

• When the data in the field changes, perhaps by moving from record to record, Microsoft Access synchronizes the data in the control with the data in the field.
• When the data in the control changes, perhaps by the user, Microsoft Access saves the data from the control to the bound field.

CAUTION: Following the steps in this example will modify the sample database Northwind.mdb. You may want to back up the Northwind.mdb file or perform these steps on a copy of the Northwind database.

1. Open the sample database Northwind, create a new module, and type the following line in the Declarations section if it isn't already there:

         Option Explicit
   

2. Type the following procedures:

         Sub mslbSyncList(ctlMSLB As Control, ctlBound As Control)
            Dim i As Integer
            Dim SourceList As String, SearchItem As String
         On Error Goto Err_mslbSyncList
            SourceList = ";" & ctlBound & ";"
            For i = 0 To ctlMSLB.ListCount - 1
               SearchItem = ";" & ctlMSLB.ItemData(i) & ";"
               ctlMSLB.Selected(i) = (InStr(SourceList, SearchItem) > 0)
            Next i
         Bye_mslbSyncList:
            Exit Sub
         Err_mslbSyncList:
            Beep
            MsgBox Error$, 16
            Resume Bye_mslbSyncList
         End Sub
   
         Sub mslbSaveList(ctlMSLB As Control, ctlBound As Control)
            Dim MyList, i
            MyList = Null
         On Error Goto Err_mslbSaveList
            For Each i In ctlMSLB.ItemsSelected
               MyList = MyList & ";" & ctlMSLB.ItemData(i)
            Next
            If Len(MyList) > 0 Then MyList = Mid(MyList, 2)
            ctlBound = MyList
         Bye_mslbSaveList:
            Exit Sub
         Err_mslbSaveList:
            Beep
            MsgBox Error$, 16
            Resume Bye_mslbSaveList
         End Sub
   
   

3. Open the Customers table in Design view. Add a new field called Employees with data type Text, and set the FieldSize property as follows:

         FieldSize: 255
   

4. Save and close the table.

5. Open the Customer form and add a text box with the following properties:

         Name: txtEmployees
         ControlSource: Employees
         Visible: No
   

   NOTE: For testing purposes, you may want to leave the Visible property set to Yes to examine the semicolon-delimited list being generated.

6. Add a new list box with the following properties:

         Name: mslbEmployees
         RowSource: Employees
         ColumnCount: 2
         ColumnWidths: 0
         MultiSelect: Simple
   

7. On the View menu, click Code to invoke the form's module and type the following procedures:

         Private Sub Form_Current()
            mslbSyncList Me![mslbEmployees], Me![txtEmployees]
         End Sub
   
         Private Sub mslbEmployees_AfterUpdate()
            mslbSaveList Me![mslbEmployees], Me![txtEmployees]
         End Sub
   
         NOTE: The mslbSyncList procedure should be called from any code that
         changes the contents of the field the semicolon-delimited list is
         stored into. The mslbSaveListfield procedure should be called from
         any code that changes the contents of the multiple-selection list box
         itself.
   
   

8. View the form in Form view, move from customer to customer, and make different selections in the new multiple-selection list box.

REFERENCES

For more information about database normalization, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q100139
   TITLE     : ACC: Database Normalization Basics

For more information about a multiple-selection, search the Help Index for "Selected."

Keywords          : kbusage
Version           : 7.0 97
Platform          : WINDOWS
Hardware          : x86
Issue type        : kbprb
Solution Type     : kbcode