SQL Server Native Client OLE DB 访问接口会话表示与 SQL Server 实例的单个连接。
SQL Server Native Client OLE DB 访问接口要求会话分隔数据源的事务空间。 从特定会话对象创建的所有命令对象都参与会话对象的本地或分布式事务。
在初始化数据源上创建的第一个会话对象接收在初始化时建立的 SQL Server 连接。 释放对会话对象的接口的所有引用时，与 SQL Server 实例的连接将可用于在数据源上创建的另一个会话对象。
在数据源上创建的其他会话对象与数据源指定的 SQL Server 实例建立自己的连接。 当应用程序释放对创建该会话的对象的所有引用时，将删除与 SQL Server 实例的连接。
以下示例演示如何使用 SQL Server Native Client OLE DB 访问接口连接到 SQL Server 数据库：
int main()
{
// Interfaces used in the example.
IDBInitialize* pIDBInitialize = NULL;
IDBCreateSession* pIDBCreateSession = NULL;
IDBCreateCommand* pICreateCmd1 = NULL;
IDBCreateCommand* pICreateCmd2 = NULL;
IDBCreateCommand* pICreateCmd3 = NULL;
// Initialize COM.
if (FAILED(CoInitialize(NULL)))
{
// Display error from CoInitialize.
return (-1);
}
// Get the memory allocator for this task.
if (FAILED(CoGetMalloc(MEMCTX_TASK, &g_pIMalloc)))
{
// Display error from CoGetMalloc.
goto EXIT;
}
// Create an instance of the data source object.
if (FAILED(CoCreateInstance(CLSID_SQLNCLI10, NULL,
CLSCTX_INPROC_SERVER, IID_IDBInitialize, (void**)
&pIDBInitialize)))
{
// Display error from CoCreateInstance.
goto EXIT;
}
// The InitFromPersistedDS function
// performs IDBInitialize->Initialize() establishing
// the first application connection to the instance of SQL Server.
if (FAILED(InitFromPersistedDS(pIDBInitialize, L"MyDataSource",
NULL, NULL)))
{
goto EXIT;
}
// The IDBCreateSession interface is implemented on the data source
// object. Maintaining the reference received maintains the
// connection of the data source to the instance of SQL Server.
if (FAILED(pIDBInitialize->QueryInterface(IID_IDBCreateSession,
(void**) &pIDBCreateSession)))
{
// Display error from pIDBInitialize.
goto EXIT;
}
// Releasing this has no effect on the SQL Server connection
// of the data source object because of the reference maintained by
// pIDBCreateSession.
pIDBInitialize->Release();
pIDBInitialize = NULL;
// The session created next receives the SQL Server connection of
// the data source object. No new connection is established.
if (FAILED(pIDBCreateSession->CreateSession(NULL,
IID_IDBCreateCommand, (IUnknown**) &pICreateCmd1)))
{
// Display error from pIDBCreateSession.
goto EXIT;
}
// A new connection to the instance of SQL Server is established to support the
// next session object created. On successful completion, the
// application has two active connections on the SQL Server.
if (FAILED(pIDBCreateSession->CreateSession(NULL,
IID_IDBCreateCommand, (IUnknown**) &pICreateCmd2)))
{
// Display error from pIDBCreateSession.
goto EXIT;
}
// pICreateCmd1 has the data source connection. Because the
// reference on the IDBCreateSession interface of the data source
// has not been released, releasing the reference on the session
// object does not terminate a connection to the instance of SQL Server.
// However, the connection of the data source object is now
// available to another session object. After a successful call to
// Release, the application still has two active connections to the
// instance of SQL Server.
pICreateCmd1->Release();
pICreateCmd1 = NULL;
// The next session created gets the SQL Server connection
// of the data source object. The application has two active
// connections to the instance of SQL Server.
if (FAILED(pIDBCreateSession->CreateSession(NULL,
IID_IDBCreateCommand, (IUnknown**) &pICreateCmd3)))
{
// Display error from pIDBCreateSession.
goto EXIT;
}
EXIT:
// Even on error, this does not terminate a SQL Server connection
// because pICreateCmd1 has the connection of the data source
// object.
if (pICreateCmd1 != NULL)
pICreateCmd1->Release();
// Releasing the reference on pICreateCmd2 terminates the SQL
// Server connection supporting the session object. The application
// now has only a single active connection on the instance of SQL Server.
if (pICreateCmd2 != NULL)
pICreateCmd2->Release();
// Even on error, this does not terminate a SQL Server connection
// because pICreateCmd3 has the connection of the
// data source object.
if (pICreateCmd3 != NULL)
pICreateCmd3->Release();
// On release of the last reference on a data source interface, the
// connection of the data source object to the instance of SQL Server is broken.
// The example application now has no SQL Server connections active.
if (pIDBCreateSession != NULL)
pIDBCreateSession->Release();
// Called only if an error occurred while attempting to get a
// reference on the IDBCreateSession interface of the data source.
// If so, the call to IDBInitialize::Uninitialize terminates the
// connection of the data source object to the instance of SQL Server.
if (pIDBInitialize != NULL)
{
if (FAILED(pIDBInitialize->Uninitialize()))
{
// Uninitialize is not required, but it fails if an
// interface has not been released. Use it for
// debugging.
}
pIDBInitialize->Release();
}
if (g_pIMalloc != NULL)
g_pIMalloc->Release();
CoUninitialize();
return (0);
}
将 SQL Server Native Client OLE DB 访问接口会话对象连接到 SQL Server 实例可能会为持续创建和释放会话对象的应用程序产生重大开销。 可以通过有效地管理 SQL Server Native Client OLE DB 访问接口会话对象来最大程度地减少开销。 SQL Server Native Client OLE DB 访问接口应用程序可以通过在至少一个对象的接口上维护引用来使会话对象的 SQL Server 连接保持活动状态。
例如，维护命令创建对象引用池可保留池中这些会话对象的活动连接。 由于需要会话对象，池维护代码会将有效的 IDBCreateCommand 接口指针传递给需要会话的应用程序方法。 当应用程序方法不再需要会话时，该方法将返回指向池维护代码的接口指针，而不是释放应用程序对命令创建对象的引用。
注释
在前面的示例中，使用 IDBCreateCommand 接口是因为 ICommand 接口实现了 GetDBSession 方法，这是命令或行集范围中唯一允许对象确定创建它的会话的方法。 因此，命令对象和命令对象仅允许应用程序检索数据源对象指针，从中可以创建其他会话。