BUG: DataMember of GetDataMember Empty When DataSource is DataList

• Microsoft Visual Basic Enterprise Edition for Windows, version 6.0

SYMPTOMS

The DataMember property of a DataList box is empty when the GetDataMember event is called, even though the DataMember property was assigned a value.

RESOLUTION

Create a Property Let in the DataSource class. In the Property Let, assign the DataMember property to a private string variable in the class. Test the DataMember property in the GetDataMember event. If the DataMember property is empty, use the value contained in the string to replace the empty DataMember property.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

MORE INFORMATION

In Visual Basic 6.0, you can create a class module that can serve as a DataSource. To create a DataSource class, set the DataSourceBehavior of a class module to 1 - vbDataSource. Specifying a class as a DataSource adds a DataMembers collection and a GetDataMember event to the class.

The DataMembers collection allows other objects to see the available DataMembers of the class. A DataMember represents a recordsets stored in the DataSource.

The GetDataMember event is called when the DataSource property of a bound object is set to the DataSource class.

The GetDataMember event has two parameters. The DataMember parameter is a string supplied by a bound object's DataMember property. The DataMember parameter selects a specific member of the class's DataMembers collection. The Data parameter of the GetDataMember event is the recordset object to be passed back to the bound object.

You should set a bound control's DataMember property before setting the control's DataSource property. That way, the control's DataMember property will supply a value for the GetDataMember's DataMember parameter before GetDataMember is called. However, when using a DataList control, the DataMember parameter is empty when GetDataMember is called, even though the DataMember property of the DataList has been assigned.

The following example demonstrates binding both a DataList box and a DataGrid to a DataSource class. When GetDataMember is called for the DataGrid, the DataMember parameter is correctly assigned. When GetDataMember is called for the DataList, the DataMember parameter is an empty string.

Setting the DataSource properties in the Form Load event calls the GetDataMember event. The GetDatamember event uses a Select Case statement, which contains an Else clause. When the DataMember is not assigned using the Property Let, the DataList and the DataGrid use different recordsets because the DataList falls through to the Else clause in the Select Case statement. If you uncomment the call to the Property Let in the Form Load event, the DataList and DataGrid will use the same recordset.

The code in Command1's Click event sets the DataGrid and the DataList to the DataMember that is default DataMember in the Case statement. Uncommenting the call to the Property Let appears to make no difference. However, in Debug mode, you can see that the DataMember is empty when called by the DataList, unless Property Let is used.

Steps to Reproduce Behavior

1. Create a standard exe project in Visual Basic and add a class module.



2. In the properties Window for the class, change DataSourceBehavior to 1 - vbDataSource.



3. Change the Name property of the class to clsNamesData.



4. Under Project References, check Microsoft Active Data Objects 2.0 Library.



5. Paste the following code in the Class code window:
   

   
   Dim rsnames As ADODB.Recordset
   Dim rsnames2 As ADODB.Recordset
   Dim alternate_datamember As String
   
   Private Sub Class_GetDataMember(DataMember As String, Data As Object)
         
     If alternate_datamember <> "" Then
         DataMember = alternate_datamember
     End If 
     alternate_datamember = ""
         
     Select Case DataMember
       Case "Names"
           Set Data = rsnames
       Case "Names2"
           Set Data = rsnames2
       Case Else
           Set Data = rsnames
     End Select
         
   End Sub
   
   Private Sub Class_Initialize()
         
      'Add the names of the new data members to the DataMembers collection.
      'This allows other objects to see the available DataMembers.
      DataMembers.Add "Names"
      DataMembers.Add "Names2"
      
      'Create the recordsets of the DataSource class.
      Set rsnames = New ADODB.Recordset
      Set rsnames2 = New ADODB.Recordset
      
      'Create the recordset schema.   
      With rsnames
         .Fields.Append "ID", adInteger
         .Fields.Append "Name", adBSTR, 255
         .CursorType = adOpenStatic
         .LockType = adLockOptimistic
         .Open
      End With
      
      'Add ten records.   
      Dim i As Integer
      For i = 1 To 10 
         rsnames.AddNew
         rsnames!ID = i
         rsnames!Name = "Name " & i
         rsnames.Update
      Next i
      rsnames.MoveFirst
      
      'Create the recordset schema.   
      With rsnames2
         .Fields.Append "ID", adInteger
         .Fields.Append "Name", adBSTR, 255
         .CursorType = adOpenStatic
         .LockType = adLockOptimistic
         .Open
      End With
      
      ' Add ten records.
      For i = 1 To 10
         rsnames2.AddNew
         rsnames2!ID = i
         rsnames2!Name = "Smith " & i
         rsnames2.Update
       Next i
      rsnames2.MoveFirst
   
   End Sub
   
   Public Property Let dmember(ByVal vNewValue As Variant)
         
      alternate_datamember = vNewValue
         
   End Property 




6. Under Project Components, check Microsoft Datagrid Control 6.0 (OLEDB) and Microsoft DataList Control 6.0 (OLEDB).



7. Add a CommandButton, a DataList box and a DataGrid to the existing form.



8. Paste the following code in the form's code window:
   

   
   Option Explicit
   Private datNames As clsNamesData
   
   Private Sub Form_Load()
     
      Set datNames = New clsNamesData
    
      DataGrid1.DataMember = "Names2"
      Set DataGrid1.DataSource = datNames
   
      ' DataMember is set here, but is "" in the class DataSource.
      DataList1.DataMember = "Names2"
       
      ' The next line is the workaround to this problem.
      ' Uncomment the following line to make it work correctly.
      ' datNames.dmember = "Names2"
       
      DataList1.ListField = "name"
      ' DataList requires RowSource instead of DataSource.    
      Set DataList1.RowSource = datNames 
       
   End Sub
   
   Private Sub Command1_Click()
       
      'Reassigning the DataGrid and DataList to another recordset.
       
      'GetDataMember is called once per control unless
      'the class is reinstantiated.
      Set datNames = Nothing
      Set datNames = New clsNamesData
       
      DataGrid1.DataMember = "Names"
      Set DataGrid1.DataSource = datNames
       
      ' DataMember is set here, but is "" in the class DataSource.
      DataList1.DataMember = "Names"
       
      ' The next line is the workaround to this problem.
      ' Uncomment the following line to make it work correctly:
      ' datNames.dmember = "Names"
       
      DataList1.ListField = "name"
      Set DataList1.RowSource = datNames
       
   End Sub 

© Microsoft Corporation 1999, All Rights Reserved.
Contributions by Margery Simms, Microsoft Corporation

REFERENCES

For instructions and sample code on how to create and bind to a DataSource class, as well as descriptions of the GetDataMember event and the DataMembers collection, please see the following topics in Microsoft Visual Basic 6.0 Online Help:

GetDataMembers event
DataMembers collection
DataGrid control, class module as data source

Additional query words:

Keywords : kbADO kbDataBinding kbVBp600bug kbGrpVBDB
Version : WINDOWS:6.0
Platform : WINDOWS
Issue type : kbbug