Q152116: SAMPLE: DAO_OCX.EXE ActiveX Control Uses DAO to Query Database

Article: Q152116
Product(s): Microsoft C Compiler
Version(s): winnt:4.1
Operating System(s): 
Keyword(s): kbfile kbsample
Last Modified: 07-MAY-2001

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The information in this article applies to:

- Microsoft Visual C++, version 4.1 
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SUMMARY
=======

This sample demonstrates querying and displaying the result of the query using
MFC's CDao classes from an ActiveX control (OLE Control.)

The DaoOcx control utilizes a saved query in the CUSTOMER.MDB database provided
with the sample. This database consists of phone numbers and their associated
addresses. Additionally, there may be several people at each address and each
individual may be associated with a note.

When the FindNumber() method is called from the client, it queries the database
for the currently-entered phone number using the saved query ByPhone, retrieves
the data, and then populates the fields with the address. It also populates a
List Box with the names of the people at that address. Once the user selects one
of the names, it places the note associated with that individual, if there is
one, in the note edit control, and then fires an event.

The client application should call the ConnectDataBase method initially and must
call the CleanUp method prior to shutdown. In the Visual Basic portion of this
sample, the ConnectDataBase call is made in the Form's Load procedure and the
CleanUp call is made in the Form's Terminate procedure.

The database name and the name of the saved query are properties of the control
and can be set. If the property sheet is used to set the properties, then
validation is done. No validation is performed if the properties are not entered
using the property sheet provided.

The following files are available for download from the Microsoft Download
Center:

Dao_ocx.exe

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.


MORE INFORMATION
================

Once you have unzipped the file, you should have the actual database,
CUSTOMER.MDB, that includes the saved query ByPhone. This database was created
using Access 2.0 and may also be accessed using Access 95. Also included are the
files for the control and a Visual Basic 4.0 (32- bit) project and form that
demonstrates how to use the control. You must build and register the control,
and then set the location of the CUSTOMER.MDB file in the DaoOcx control on the
Visual Basic form before running the Visual Basic client.

Once the client is running, enter one of the following phone numbers (555)
555-5555, (526) 874-4632 or (816) 547-1234. Click the Find button to see some
sample data. Once the names are visible, select one and see the note associated
with that individual. You will also see a message box caused by an event fired
from the control indicating that the data is complete.

The control uses a CDaoRecordSet derived class that is used to query the
database. This class can be generated by hand, or you can use the AppWizard to
create it as follows:

1. From the File menu, select New. Select Project Workspace, and then select MFC
  AppWizard.

2. Select an MDI application and Database support.

3. Press the Data Source button.

4. Select the DAO button.

5. Select the CUSTOMER.MDB file in the project directory as the database.

AppWizard will create a CDaoRecordSet derived class that can be used almost
directly. The only changes needed to be made to the CDaoRecordSet derived class
are as follows:

1. Modify the constructor to take a pointer to the control.

2. Modify the GetDefaultDBName() function to call the control and retrieve the
  database name.

3. Modify the GetDefaultSQL() function to call the control and retrieve the
  query name.

When Dao is used in a DLL, you must explicitly call AfxDaoTerm() prior to the
call to ExitInstance() or you will receive an assertion. In the sample, this
behavior is accomplished by calling the automation method, CleanUp, that closes
any of the database access items and calls AfxDaoTerm(). MFC will call
AfxDaoInit() to initialize the Jet Engine, but because you are calling from a
DLL, the "CleanUp" method must call AfxDaoTerm() directly.

The CDao classes are very similar in usage to the ODBC database classes except
that the Dao classes only access the Jet Engine, while the ODBC classes use the
ODBC layer to allow access to a wide range of database types.

The CDao classes are wrappers around the Dao COM interfaces. For example, the
CDaoQueryDef is a wrapper for the DAOQueryDef interface and the class has a
member variable, m_pDAOQueryDef, that is a pointer to the interface.

Each object in the hierarchy contains an enumeration interface for listing the
next lower interface in the hierarchy. For example, m_pDAOQueryDef
(DAOQueryDef*) can be used to call QueryInterface for the DAOFields interface
used to enumerate the fields in that query. The DAODatabase interface can be
used to get the DAOQueryDefs interface used to list the available queries in the
database.

Calling Open on a variable of type CDaoRecordSet causes the SQL query to be
generated and the field variables to be filled.

You must call Close on any CDao object (interface) when you are done using it.

Additional query words: 4.10

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Keywords          : kbfile kbsample 
Technology        : kbVCsearch kbAudDeveloper kbVC410
Version           : winnt:4.1

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