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Visual Basic Tutorial Part 7

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use in any application.
Why are there three data access interfaces in Visual
Basic? Data access technology is constantly evolving,
and each of the three interfaces repres

nt a different
state of the art. The latest is ADO, which features a
simpler -- yet more flexible -- object model than either
RDO or DAO. For new projects, you should use ADO as
your data access interface.
Why Use ADO?
ADO is designed as an easy-to-use application level
interface to Microsoft's newest and most powerful data
access paradigm, OLE DB. OLE DB provides
high-performance access to any data source, including
relational and non-relational databases, email and
file systems, text and graphics, custom business
objects, and more. ADO is implemented for minimal
network traffic in key Internet scenarios, and a
minimal number of layers between the front-end and
data source -- all to provide a lightweight,
high-performance interface. ADO is called using a
familiar metaphor -- the OLE Automation interface. And
ADO uses conventions and features similar to DAO and
RDO, with simplified semantics that make it easy to
learn.
For a brief overview, see OLE DB Providers.
For detailed information about ADO, see Getting
Started with ADO.
DAO and RDO
For backward compatibility, Visual Basic continues to
support DAO and RDO for existing projects.
For More Information For more information on RDO
programming, see Using Remote Data Objects and the
RemoteData Control. For information on DAO
programming, see Using Data Access Objects with Remote
Databases. Complete DAO reference can also be found at
Microsoft DAO 3.6.
Upgrading from RDO to ADO
Consider upgrading if you decide ADO offers benefits
your RDO-based application can use. See ADO Compared
with RDO and DAO for a discussion of the differences
among the platforms and for guidance on changing an
RDO-based project to an ADO project. See Converting
from RDO 2.0 to ADO 2.0 for upgrade guidance.

Send feedback to MSDN. Look here for MSDN Online
resources.

ADO Cursor Library

The ActiveX Data Objects (ADO) cursor provider, part
of the Remote Data Service (RDS) technology, provides
several types of cursors as shown in the following
table.
Cursor type
Constant
Forward-only cursor
adOpenForwardOnly
Keyset-driven cursor
adOpenKeyset
Dynamic cursor adOpenDynamic


Static cursor adOpenStatic

You can control how the data source and the chosen ADO
cursor library manage concurrency with the locking
options in the following table.
Locking type
Constant
Pessimistic concurrency.
adLockPessimistic
Optimistic concurrency using row values.
adLockOptimistic
Read-only. Changes are not permitted. adLockReadOnly
All updates are deferred until the batch update is
finished. To do batch updating, you should select
either a keyset or static cursor.
adLockBatchOptimistic

For More Information For more information on ADO
cursor options, search online for "CursorType
Property" in MSDN Library Visual Studio 6.0. For more
information on Remote Data Service (RDS) and how it
provides cursor support with ActiveX Data Objects,
search online for "Remote Data Service Developer's
Guide" and "Understanding Remote Data Service
Applications" in MSDN Library Visual Studio 6.0. For
more information on using locks to handle multiuser
concurrency situations, see Managing Concurrency with
Cursor Locks in this chapter.

Send feedback to MSDN. Look here for MSDN Online
resources.




39. What is the difference between a ActiveX DLL and a
ActiveX EXE?

ActivexDLL is executes in in process

ActivexExe executes in outprocess


40. What are the types of Instancing property that can
be set for a Class in a ActiveX DLL and ActiveX EXE?
Visual Basic Concepts
Instancing for Classes Provided by ActiveX Components

The value of the Instancing property determines
whether your class is private -- that is, for use only
within your component -- or available for other
applications to use.
As its name suggests, the Instancing property also
determines how other applications create instances of
the class. The property values have the following
meanings.

Private means that other applications aren't allowed
access to type library information about the class,
and cannot create instances of it. Private objects are
only for use within your component.


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