Microsoft KB Archive/170163

= INFO: VB 6.0 Readme Part 2: Data Access/Databinding Issues =

Article ID: 170163

Article Last Modified on 5/8/2003

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APPLIES TO


 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition

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This article was previously published under Q170163



SUMMARY
The information below includes the documentation and workarounds for Visual Basic 6.0. This information can also be found in the README.htm file that ships with Visual Basic 6.0 on the Visual Basic 6.0 CD-ROM. Please see the REFERENCES section of this article for a list of the Microsoft Knowledge Base articles relating to the Visual Basic 6.0 readme.

Following is a list of all parts of the readme file:

Part 1. Important Issues - Please Read First!

Part 2. Data Access Issues and DataBinding Tips

Part 3. Control Issues

Part 4. Language Issues

Part 5. Samples Issues

Part 6. Wizard Issues

Part 7. Error Message Issues

Part 8. WebClass Designer Issues

Part 9. DHTML Page Designer Issues

Part 10. Extensibility issues

Part 11. Miscellaneous Issues

Part 12. Microsoft Transaction Server (MTS) Issues

Part 13. Dictionary Object

Part 14. Visual Component Manager

Part 15. Application Performance Manager



Error in Data Environment Designer Code Example
In the topic, "Programmatically Accessing Objects in Your Data Environment Designer," the example under "Executing a Command Object with Multiple Parameters" erroneously uses the Open method:

MyDE.Commands("InsertCustomer").Parameters("ID").value = "34" MyDE.Commands("InsertCustomer").Parameters("Name").value = "Fred" MyDE.Commands("InsertCustomer").Open

There is no Open method for the Commands object. You must use the Execute method instead.

Incompatibilities with Data-bound Controls
Due to changes in Visual Basic 6.0, not all data-bound controls are compatible with all data sources. This incompatibility is due to a difference in the internal binding mechanisms of ADO versus DAO/RDO. Controls that were created specifically to work with DAO/RDO can't be bound to an ADO Data control; controls created for use with ADO can't be bound to the standard Data control or the Remote Data Control.

This incompatibility primarily applies to complex-bound controls such as grids or lists that bind to multiple fields in a data source; simple-bound controls such as text boxes or labels that bind to a single field will work with either type of data source. Some examples are as follows:


 * 1) The Microsoft Data Bound Grid control (Dbgrid32.ocx) can be bound to the DAO or RDO Data controls; it can't be bound to the ADO Data control.
 * 2) The Microsoft DataGrid control (Msdatgrd.ocx) can be bound to the ADO Data control; it can't be bound to the DAO or RDO Data controls.
 * 3) The Microsoft Masked Edit Control (Msmask32.ocx) can be bound to any of the Data controls.
 * 4) The intrinsic controls (TextBox, PictureBox, Label, and so on) can be bound to any of the Data controls.
 * 5) Third-party controls and Visual Basic-authored User controls should be tested on a case-by-case basis.

When attempting to bind a control to a data source at design-time, you may encounter a "No compatible data source" error message. In this case, you will need to substitute another control that is compatible with your data source.

Binding to Properties of Objects May Yield Unexpected Results
While it is possible to bind any object to any other object, the results may not always be what you expect. Some properties are read-only bindable and will not update their bound source.

For example, if you were to bind the Caption property of a Frame control to a field named Foo in an ADO Recordset object, the Caption would change to reflect the value of Foo as you scrolled through the Recordset. If, however, you changed the Caption property programmatically (Frame1.Caption = "Bar"), the value of Foo would not be updated. Because the Caption property of the Frame is read-only bindable, it doesn't provide notification that its data has changed.

This isn't a problem for Visual Basic-authored objects, since you can call the PropertyChanged method in your object's code. For other objects, you can determine if a property is update bindable by checking the DataBindings collection. If a property is enumerated in the DataBindings collection, it is update bindable and the data source will receive updates to data; if it isn't enumerated, the property is read-only bindable.

Complex Binding to an ADO Recordset Requires CursorType
When binding an ADO Recordset object to a complex-bound control (such as a Grid control), it is necessary to explicitly set the CursorType property to either adOpenStatic or adOpenKeyset. If you don't set this property, no data will be displayed. The following code shows the use of the CursorType property:

Private Sub DataClass_Initialize Set cn = New ADODB.Connection Set rs = New ADODB.Recordset rs.CursorType = adOpenStatic cn.Open "northwind" rs.Open "customers", cn End Sub Binding to a simple-bound control (such as a TextBox) doesn't require a specific CursorType.

Creating Visual Basic Data Sources: Type the Fields as adVarChar

for SQL Server and Access Databases Instead of adBSTR
When appending fields to an ADO Recordset object for use with a SQL Server or Access database, type the fields as adVarChar instead of adBSTR (as shown in some sample code). When reading data out of either SQL Server or Access databases, ADO will use the adVarChar type.

Incorrect References for Creating OLE DB Providers
The documentation erroneously states that it is possible to set a class module's DataSourceBehavior property to 2 - vbOLEDBProvider to create an OLE DB data provider. The correct values for DataSourceBehavior are 0 - vbNone and 1 - vbDataSource.

The documentation also erroneously refers to a non-existent event in class modules called OnDataConnection.

Finally, in the topic "Creating the MyDataSource Class," the step-by-step example incorrectly states that you should set DataSourceBehavior to 2 - vbOLEDBProvider. Instead, you should set DataSourceBehavior to 1 - vbDataSource.

To create OLE DB data providers using Visual Basic, use the Provider Writer Toolkit included with the OLE DB SDK. For more information, see the OLE DB Simple Provider Toolkit in the Platform SDK Documentation on MSDN.

Finding Help For ADO Objects
When using the ADO objects, (for example, Recordset, Connection, Command, Parameter, ADOR, RDS, and RDS Server object), you cannot get context- sensitive help on the object or its properties, events, or methods. That is, if you have a reference to the object and you use one of its features, selecting the code and pressing F1 does not result in a help topic. Instead, you will get either a wrong topic or the "Keyword Not Found" topic.

However, you can get help on any of the object's properties, events, or methods by using the online documentation Index:


 * 1) If the MSDN documentation viewer is not open, on the Help menu, click Contents.
 * 2) Click the Index tab.
 * 3) Type the name of the property, event, or method including the word "collection", "property", "event", or "method" as appropriate.
 * 4) From the list of available topics, select the topic that includes "ADO" in its title.

NOTE: You can also find additional help on other ADO topics, such as the ADO object model, by looking in the MSDN Library Table of Contents: open Platform SDK and under Database and Messaging Services, go to Microsoft Data Access SDK.

SQL Server OLE DB Provider Requires New instcat.sql
Before using the SQL Server OLE DB data provider, you must run the version of instcat.sql distributed with Microsoft Visual Basic 6.0 on SQL Server (version 6.5 and later). Instcat.sql is distributed with Visual Basic 6.0 and can be found in the \winnt\system32 directory upon installation.

If Instcat.sql is not run on your SQL Server, the provider is unable to retrieve metadata from the SQL Server, and thus will not be able to connect to that server.

Setup for Data Access Applications May Fail on Windows 95/98
When redistributing a Visual Basic 6.0 application that includes data access components, setup will fail if DCOM for Windows 95 and Windows 98 isn't present on Windows 9x client machines.

The file Mdac_typ.exe is added to your setup package by the Package and Deployment Wizard if your project includes references to ADO, OLEDB, or ODBC (you can check for this on the Included Files page of the wizard). This file installs MDAC 2.0 files on the client computer. MDAC 2.0 requires DCOM for Windows 95 and Windows 98 in order to function properly, however it does not perform a check for this during setup. The setup will fail if DCOM for Windows 95 and Windows 98 isn't present on the client machine. Some of the older data access components will be overwritten prior to the failure, possibly causing older data access applications on the client to fail.

When distributing data access applications for Windows 9x, you need to make sure that DCOM for Windows 95 and Windows 98 is installed on the client. DCOM98.EXE is a self-extracting executable file that installs the updated DCOM components for Windows 95 or Windows 98. It can be found in the DCOM98 directory of the Visual Basic 6.0 CD. This file may be freely distributed with your Visual Basic application.

