Manages a named, password-protected database session from login to logoff, by a single user.
In most cases, you will not need multiple workspaces, and you will not need to create explicit workspace objects; when you open database and recordset objects, they use DAO's default workspace. However, if needed, you can run multiple sessions at a time by creating additional workspace objects. Each workspace object can contain multiple open database objects in its own Databases collection. In MFC, a workspace is primarily a transaction manager, specifying a set of open databases all in the same "transaction space."
The DAO database classes are distinct from the MFC database classes based on Open Database Connectivity (ODBC). All DAO database class names have a "CDao" prefix. In general, the MFC classes based on DAO are more capable than the MFC classes based on ODBC. The DAO-based classes access data through the Microsoft Jet database engine, including ODBC drivers. They also support Data Definition Language (DDL) operations, such as creating databases and adding tables and fields via the classes, without having to call DAO directly.
Class CDaoWorkspace provides the following:
Explicit access, if needed, to a default workspace, created by initializing the database engine. Usually you use DAO's default workspace implicitly by creating database and recordset objects.
A transaction space in which transactions apply to all databases open in the workspace. You can create additional workspaces to manage separate transaction spaces.
An interface to many properties of the underlying Microsoft Jet database engine (see the static member functions). Opening or creating a workspace, or calling a static member function before open or create, initializes the database engine.
Access to the database engine's Workspaces collection, which stores all active workspaces that have been appended to it. You can also create and work with workspaces without appending them to the collection.
MFC does not implement the Users and Groups collections in DAO, which are used for security control. If you need those aspects of DAO, you must program them yourself via direct calls to DAO interfaces. For information, see Technical Note 54.
You can use class CDaoWorkspace to:
Explicitly open the default workspace.
Usually your use of the default workspace is implicit — when you open new CDaoDatabase or CDaoRecordset objects. But you might need to access it explicitly — for example, to access database engine properties or the Workspaces collection. See "Implicit Use of the Default Workspace" below.
Create new workspaces. Call Append if you want to add them to the Workspaces collection.
Open an existing workspace in the Workspaces collection.
Creating a new workspace that does not already exist in the Workspaces collection is described under the Create member function. Workspace objects do not persist in any way between datababase engine sessions. If your application links MFC statically, ending the application uninitializes the database engine. If your application links with MFC dynamically, the database engine is uninitialized when the MFC DLL is unloaded.
Explicitly opening the default workspace, or opening an existing workspace in the Workspaces collection, is described under the Open member function.
End a workspace session by closing the workspace with the Close member function. Close closes any databases you have not closed previously, rolling back any uncommitted transactions.
DAO manages transactions at the workspace level; hence, transactions on a workspace with multiple open databases apply to all of the databases. For example, if two databases have uncommitted updates and you call CommitTrans, all of the updates are committed. If you want to limit transactions to a single database, you need a separate workspace object for it.
Implicit Use of the Default Workspace
MFC uses DAO's default workspace implicitly under the following circumstances:
If you create a new CDaoDatabase object but do not do so through an existing CDaoWorkspace object, MFC creates a temporary workspace object for you, which corresponds to DAO's default workspace. If you do so for multiple databases, all of the database objects are associated with the default workspace. You can access a database's workspace through a CDaoDatabase data member.
Similarly, if you create a CDaoRecordset object without supplying a pointer to a CDaoDatabase object, MFC creates a temporary database object and, by extension, a temporary workspace object. You can access a recordset's database, and indirectly its workspace, through a CDaoRecordset data member.
Other database operations are also provided, such as repairing a corrupted database or compacting a database.
For information about calling DAO directly and about DAO security, see Technical Note 54.