For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.
Represents an exception condition arising from the MFC database classes based on data access objects (DAO).
class CDaoException : public CException
|CDaoException::CDaoException||Constructs a |
|CDaoException::GetErrorCount||Returns the number of errors in the database engine's Errors collection.|
|CDaoException::GetErrorInfo||Returns error information about a particular error object in the Errors collection.|
|CDaoException::m_nAfxDaoError||Contains an extended error code for any error in the MFC DAO classes.|
|CDaoException::m_pErrorInfo||A pointer to a CDaoErrorInfo object that contains information about one DAO error object.|
|CDaoException::m_scode||The SCODE value associated with the error.|
The class includes public data members you can use to determine the cause of the exception.
CDaoException objects are constructed and thrown by member functions of the DAO database classes.
The DAO database classes are distinct from the MFC database classes based on Open Database Connectivity (ODBC). All DAO database class names have the "CDao" prefix. You can still access ODBC data sources with the DAO classes. In general, the MFC classes based on DAO are more capable than the MFC classes based on ODBC; the DAO-based classes can access data, including through ODBC drivers, via their own database engine. The DAO-based classes also support Data Definition Language (DDL) operations, such as adding tables via the classes, without having to call DAO directly. For information on exceptions thrown by the ODBC classes, see CDBException.
In MFC, all DAO errors are expressed as exceptions, of type
CDaoException. When you catch an exception of this type, you can use
CDaoException member functions to retrieve information from any DAO error objects stored in the database engine's Errors collection. As each error occurs, one or more error objects are placed in the Errors collection. (Normally the collection contains only one error object; if you are using an ODBC data source, you are more likely to get multiple error objects.) When another DAO operation generates an error, the Errors collection is cleared, and the new error object is placed in the Errors collection. DAO operations that do not generate an error have no effect on the Errors collection.
For DAO error codes, see the file DAOERR.H. For related information, see the topic "Trappable Data Access Errors" in DAO Help.
For more information about exception handling in general, or about
CDaoException objects, see the articles Exception Handling (MFC) and Exceptions: Database Exceptions. The second article contains example code that illustrates exception handling in DAO.
Ordinarily, the framework creates exception objects when its code throws an exception. You seldom need to construct an exception object explicitly. If you want to throw a
CDaoException from your own code, call the global function AfxThrowDaoException.
However, you might want to explicitly create an exception object if you are making direct calls to DAO via the DAO interface pointers that MFC classes encapsulate. In that case, you might need to retrieve error information from DAO. Suppose an error occurs in DAO when you call a DAO method via the DAODatabases interface to a workspace's Databases collection.
Call the exception object's GetErrorCount member function to determine how many error objects are in the database engine's Errors collection. (Normally only one, unless you are using an ODBC data source.)
Call the exception object's GetErrorInfo member function to retrieve one specific error object at a time, by index in the collection, via the exception object. Think of the exception object as a proxy for one DAO error object.
In the case of an ODBC data source, repeat steps 3 and 4 as needed, for more error objects.
If you constructed the exception object on the heap, delete it with the delete operator when you finish.
For more information about handling errors in the MFC DAO classes, see the article Exceptions: Database Exceptions.
Call this member function to retrieve the number of DAO error objects in the database engine's Errors collection.
The number of DAO error objects in the database engine's Errors collection.
This information is useful for looping through the Errors collection to retrieve each of the one or more DAO error objects in the collection. To retrieve an error object by index or by DAO error number, call the GetErrorInfo member function.
Normally there is only one error object in the Errors collection. If you are working with an ODBC data source, however, there could be more than one.
Returns error information about a particular error object in the Errors collection.
void GetErrorInfo(int nIndex);
The index of the error information in the database engine's Errors collection, for lookup by index.
Call this member function to obtain the following kinds of information about the exception:
GetErrorInfo stores the information in the exception object's
m_pErrorInfo data member. For a brief description of the information returned, see m_pErrorInfo. If you catch an exception of type
CDaoException thrown by MFC, the
m_pErrorInfo member will already be filled in. If you choose to call DAO directly, you must call the exception object's
GetErrorInfo member function yourself to fill
m_pErrorInfo. For a more detailed description, see the CDaoErrorInfo structure.
For information about DAO exceptions, and example code, see the article Exceptions: Database Exceptions.
Contains an MFC extended error code.
This code is supplied in cases where a specific component of the MFC DAO classes has erred.
Possible values are:
NO_AFX_DAO_ERROR The most recent operation did not result in an MFC extended error. However, the operation could have produced other errors from DAO or OLE, so you should check m_pErrorInfo and possibly m_scode.
AFX_DAO_ERROR_ENGINE_INITIALIZATION MFC could not initialize the Microsoft Jet database engine. OLE might have failed to initialize, or it might have been impossible to create an instance of the DAO database engine object. These problems usually suggest a bad installation of either DAO or OLE.
AFX_DAO_ERROR_DFX_BIND An address used in a DAO record field exchange (DFX) function call does not exist or is invalid (the address was not used to bind data). You might have passed a bad address in a DFX call, or the address might have become invalid between DFX operations.
AFX_DAO_ERROR_OBJECT_NOT_OPEN You attempted to open a recordset based on a querydef or a tabledef object that was not in an open state.
Contains a pointer to a
CDaoErrorInfo structure that provides information on the DAO error object that you last retrieved by calling GetErrorInfo.
This object contains the following information:
|m_lErrorCode||Error Code||The DAO error code|
|Source||The name of the object or application that originally generated the error|
|Description||A descriptive string associated with the error|
|Help File||A path to a Windows Help file in which the user can get information about the problem|
|m_lHelpContext||Help Context||The context ID for a topic in the DAO Help file|
For full details about the information contained in the
CDaoErrorInfo object, see the CDaoErrorInfo structure.
Contains a value of type
SCODE that describes the error.
This is an OLE code. You will seldom need to use this value because, in almost all cases, more specific MFC or DAO error information is available in the other
CDaoException data members.
For information about
SCODE, see the topic Structure of OLE Error Codes in the Windows SDK. The
SCODE data type maps to the
HRESULT data type.