Q177425: INFO: Replacing #import’s Exception Raising Mechanism for ADO

Article: Q177425
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbATM kbCompiler kbDatabase kbVC kbGrpDSVCDB kbGrpDSMDAC kbDSupport kbMDACNoSweep
Last Modified: 26-JUL-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
- Microsoft Visual C++.NET (2002) 
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SUMMARY
=======

Using the #import statement to create your client application introduces
exception handling through the _com_error exception class when a wrapper for an
object's method encounters a failed HRESULT. You might have valid reasons to
replace this mechanism with your own implementation.

MORE INFORMATION
================

There are two ways to use #import and not have it raise exceptions for failed
HRESULTS. The first is simply to use the raw_interfaces_only clause with the
#import statement. However, this negates some of the advantages of the wrapper
classes that #import provides.

The second technique is by providing your own implementation for
_com_raise_error, which has the following prototype and default implementation:

     void __stdcall _com_raise_error(HRESULT hr, IErrorInfo* perrinfo = 0)
     throw(_com_error);

     void __stdcall
     _com_raise_error(HRESULT hr, IErrorInfo* perrinfo = 0) throw(_com_error)
     {
         throw _com_error(hr, perrinfo);
     }

This function is declared but not implemented in the Comdef.h file. If you
provide your own implementation in an .OBJ file, the linker uses that
implementation as opposed to bringing it in from the Comsupp.lib file.
_com_raise_error exists in its own object in the Comsupp.lib file so it can be
easily replaced by your code.

Following is a sample implementation of the #import's exception raising
function.

NOTE: Currently the compiler ignores a function exception-specification and
generates the following warning:

  warning C4290: C++ Exception Specification ignored.

At this time, the implementation details of exception specification have not been
standardized, and are accepted but not implemented in Microsoft Visual C++. Code
compiled with ignored exception specifications may need to be recompiled and
linked to be reused in future versions supporting exception specifications. You
can avoid this warning by using the warning pragma:

  #pragma warning( disable : 4290 ) 

Sample Code
-----------

     void __stdcall
     _com_raise_error(HRESULT hr, IErrorInfo* perrinfo = 0) throw(_com_error)
         {
             //This message box is for demonstration purpose only.
             AfxMessageBox( "_com_raise_error (HRESULT, IErrorInfo*)" );
             //Your own error handling code or just an abort.
         }

      #import <msado15.dll>
          ...

       _bstr_t     bstrEmpty(L"");
       _ConnectionPtr  Conn1 = NULL;
       Conn1.CreateInstance( __uuidof( Connection ) );
       Conn1->Open( bstrEmpty, bstrEmpty, bstrEmpty );

This code attempts to open an ActiveX Data Objects (ADO) connection object
without providing any valid connection information. Replacing _com_raise_error
prevented the _com_error from being raised.

Additional query words:

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Keywords          : kbATM kbCompiler kbDatabase kbVC kbGrpDSVCDB kbGrpDSMDAC kbDSupport kbMDACNoSweep 
Technology        : kbVCsearch kbAudDeveloper kbVC500 kbVC600 kbVC32bitSearch kbVCNET kbVC500Search
Version           : :5.0,6.0
Issue type        : kbinfo

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