INFO: Using ActiveX Data Objects (ADO) via #import in VC++

The #import directive in Visual C++ offers a powerful new mechanism for manipulating OLE servers. When used with ActiveX Data Objects (ADO), #import can simplify getting at your data. This article discusses what is necessary to take advantage of #import with ADO.

Before You Instantiate Any Classes Created by #import

It's important to initialize OLE before creating any instances of classes created by #import. For example, the following code is safe, as it declares a #import smart pointer, initializes OLE, and then instantiates the smart pointer:

The next code sample, however, is not safe and generates an unhandled exception. The global smart pointer p is both declared and instantiated (by virtue of passing a specific uuid in the constructor):

Because p is a global variable, it is instantiated before CoInitialize is ever called in main(). You can correct this with the following code snippet :
An instance of the struct InitOle is declared, and instantiated before p, and, therefore, initializes OLE in it's constructor. Without this kind of fail-safe, you will see the following error message:

Correct Implementation of #import

It is important to invoke ADO correctly in your program, or you can have compiler errors. The following code demonstrates the correct way to use #import with Msado10.dll the MSADO15.dll:

Error Handling

With ADO, you may get an error in the HRESULT returned from an ADO method, you may get an exception raised by #import generated classes, and for either condition the ADO Errors Collection may be populated. In order to get at the Errors Collection you need a valid connection object. For more information please see the following article in the Microsoft Knowledge Base:

ADO and Dbdaoint.h

Attempts to mix ADO (through #import) and either MFC DAO or the DAO SDK in the same implementation file, as follows:


Generates the following six errors:

While very nearly identical in content, the actual values in each enumerated type differ between what is required by ADO and what is required by DAO. You have several options to work around this:

• Separate ADO and DAO code into separate .cpp files. Keep the use of #import or #include <afxdao.h/dbdao.h> in separate implementation files as well.
• Modify the #import statement to create a namespace for anything generated for ADO. This means you will have to reference the namespace when referencing an ADO object as shown in the two functions below. The first shows how to use ADO exclusively within a function. The second shows how to mix-and-match ADO and DAO objects. This is possible only by explicitly referencing the ADO namespace for any ADO class or enumerated type:
  
  

     #include <afxdao.h>
  
        #import <msado15.dll>                            \ 
                rename_namespace("AdoNS") rename( "EOF", "adoEOF" )
  
        void ADOOnly( void )
        {
            using namespace AdoNS;
  
            _RecordsetPtr   prs;
  
            // Generates Compile Errors:
            CDaoRecordset   rs;
        }
  
        void MixAdoAndDao( void )
        {
            AdoNS::_RecordsetPtr  prs;
  
            // Compiles just fine
            CDaoRecordset   drs;
        }
  						

Dissecting and using Msado105.tlh/Msado15.tli --------------------------------------------

#import generates two files, Msado105.tlh and Msado15.tli off of the typelib contained within Msado15.dll. The structure of the .tlh file can be broken out as follows:

• Forward References and Typedefs
• Smart Pointer Typedef and Declarations
• Type Library Items

Each is described in detail below.

Forward References and Typedefs

Forward References and Typedefs are created through the use of struct __declspec(uuid("...")) on the GUID for any Dual Interface, Interface, and CoClass defined in the typelib.

Not all interfaces, such as Connection, have multiple implementations. This depends on the typelib, but for ADO most interfaces are dual and not implemented as interface or coclass.

Smart Pointer TypeDef Declarations

For Interfaces and Dual Interfaces, smart pointers are declared, which greatly simplifies using the interface: Note that no smart pointer was declared for the coclass Connection interface.

Type Library Items

This includes any enumerated types defined in the typelib, as well implementation of the smart pointers and typelib items:

In the preceding code fragment, the Property Data section uses declspec to declare get and put methods for ConnectionString. The Wrapper methods section provides methods created by #import, which wrap these methods, and raise an _com_error exception if they are not successful. The Raw Methods section declares the actual method that is invoked by the interface.

While you could call GetConnectionString or PutConnectionString, it is really unnecessary. Since ConnectionString is a property you would reference it as follows:



The actual implementation of GetConnectionString/PutConnectionString can be found in the Msado15.tli file.

When it comes time to use the Connection object in your code, you would use an instance of the smart pointer for the dual interface defined in Msado15.tlh as follows:



Where pubs is an ODBC data source.

#import and Explicitly Calling Release()

The advantage of #import is that it takes care of AddRef, QueryInterface, and Release for you automatically. However, if you decide to start calling Release() explicitly, you can create problems for yourself.

Within _com_ptr_t is a member variable, m_pInterface. As #import is a very thin wrapper, it makes no distinction with m_pInterface after the object is actually released, versus just decrementing its reference count without actually destroying the object. By explicitly calling Release()--without very explicitly calling AddRef() to balance it--#import will gladly try to release an object that doesn't exist, creating interesting side effects and crashing behavior.

Best advice, you did not AddRef() it (or at least no need to), do not release it either.

• Inside Com by Dale Rogerson ISBN 1-57231-349-8
• The OLE-COM Object Viewer (Oleview.exe) that ships with Visual C++ for examining the contents of a typelib.
• Visual C++ online documentation: search on #import

For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base: