Re: Calling COM functions using IDispatch->Invoke(...

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
microsoft.public.vc.mfc
Date:
Tue, 22 Apr 2008 17:47:36 +0200
Message-ID:
<XvKdnTABzNAGlZPVnZ2dnUVZ_hWdnZ2d@comnet>
* Alex:

I?ve got to call some COM server functions. It appears, that the only
way I can do it ? using IDispatch. After I?ve imported type library I
can see the prototypes for some functions. For example:
HRESULT Start( _bstr_t Path, const _variant_t & Param );

Here is my code:

HRESULT hr=1;
IDispatch *pIDispatch = NULL;
CLSID clsid;
CLSIDFromProgID(L" SomeCOM.SomeInterface ", &clsid);

hr = CoInitialize(NULL);

if( SUCCEEDED(hr) )
(
      hr = CoCreateInstance( clsid,
                 NULL, CLSCTX_LOCAL_SERVER,
                 IID_IDispatch,
                 (void**)&pIDispatch
               );

if( SUCCEEDED(hr) )
{
    DISPID dispid;
    OLECHAR FAR* szMemberName = L"Start";

   hr = pIDispatch->GetIDsOfNames(IID_NULL, &szMemberName, 1,
 
LOCALE_SYSTEM_DEFAULT, &dispid);

  if( SUCCEEDED(hr) )
  {
     VARIANT varResult;
     DISPID dispidNamed = DISPATCH_METHOD;
     EXCEPINFO excep;
     UINT uArgErr;

     DISPPARAMS dispparams;
     dispparams.cNamedArgs = 0;
     dispparams.cArgs = 2;
     dispparams.rgdispidNamedArgs = &dispidNamed;

     dispparams.rgvarg = new VARIANTARG[2];

     dispparams.rgvarg[0].vt = VT_ERROR;
     dispparams.rgvarg[0].lVal = DISP_E_PARAMNOTFOUND;

     dispparams.rgvarg[1].vt = VT_BSTR;
     dispparams.rgvarg[1].bstrVal = SysAllocString(L?SomePath?);

     hr = pIDispatch->Invoke( dispid, IID_NULL,
    LOCALE_SYSTEM_DEFAULT,
   DISPATCH_METHOD,
                 &dispparams,
                 &varResult,
                 &excep,
                 &uArgErr
              );

     if( SUCCEEDED(hr) )
     {
         //invoke succeeded
         ..........
     }
     else
     {
         //invoke failed

         // I'm getting in here ..
     }
  }

}

And I?m always getting invoke failed.
GetLatestError() returns 0.

If I?m using Alf?s P. Steinbach code, which he very kindly supplied in
my previous post, for some wrapper class for IDispatch, calling of
this method also fails, but I?m able to get some strange error,
something like ??String is too long??

So is possible to tell me what I?m doing wrong? Am I setting
"dispparams" incorrectly?


Don't know. Looks OK, including the backwards indexing. But then, it isn't
always necessary to know what the problem is in order to fix it! :-)

By the way in VB it works just fine as

Dim oI As Object
oI = CreateObject("SomeCOM.SomeInterface")
oI. Start(?SomePath?)

But I?m using STL library for handling of the data, so I really would
like to be able to call this COM functions directly from my C++
application.


Happily I hadn't yet thrown away the code I posted last time (it's an
all-purpose VC project I use for examples in e.g. Usenet postings), and with a
little added functionality -- destructor and an "addParam" function in
DispatchParameters class, I think it deals with your current problem.

Before blindly using this, though, see the comment in ~DispatchParameters.

Sorry about the linebreaks if they're broken in the Usenet posting.

<code file="OleAutomationObject.hpp">
#ifndef OLE_AUTOMATION_OBJECT
#define OLE_AUTOMATION_OBJECT

#include "wrapper/windows_h.h"
#include <ole2.h> // IDispatch
#include <comdef.h> // _com_ptr_t, _variant_t, _bstr_t
#include <vector>

#include <stdexcept>
#include <string>

namespace alfs
{
     namespace detail
     {
         // Exception helper, should really be in a file of its own:
         bool throwX( char const s[] ) { throw std::runtime_error( s ); }

         std::string narrow( wchar_t const wide[] )
         {
             std::string result( 2*wcslen(wide), '\0' );
             size_t const n = wcstombs( &result[0], wide, result.size() );
             (n != size_t(-1)) || throwX( "alfs::narrow: failed" );
             result.resize( n );
             return result;
         }

         bool throwHRX( char const s[], HRESULT hr )
         {
             throw std::runtime_error( s + narrow( _com_error( hr
).ErrorMessage() ) );
         }

         _COM_SMARTPTR_TYPEDEF( IDispatch, __uuidof(IDispatch) ); // IDispatchPtr

     } // namespace detail

     class DispatchParams
     {
     private:
         std::vector<VARIANTARG> myArgValues;
         std::vector<DISPID> myNamedArgIds;
         DISPPARAMS myParams;

         DispatchParams( DispatchParams const& ); // No such.
         DispatchParams& operator=( DispatchParams const& ); // No such.

     public:
         static VARIANTARG defaultParam()
         {
             VARIANTARG v = {};

             v.vt = VT_ERROR;
             v.scode = DISP_E_PARAMNOTFOUND;
             return v;
         }

         DispatchParams( int nParameters )
             : myArgValues( nParameters == 0? 1 : nParameters, defaultParam() )
             , myNamedArgIds( 1 )
         {
             myParams.rgvarg = &myArgValues[0];
             myParams.rgdispidNamedArgs = &myNamedArgIds[0];
             myParams.cArgs = nParameters;
             myParams.cNamedArgs = 0;
         }

         ~DispatchParams()
         {
             // Not really sure if this should be done or is callee's
responsibility,
             // and I'm too lazy to check that just for a Usenet posting!
Anyway, it
             // seems to work.
             for( size_t i = 0; i < myArgValues.size(); ++i )
             {
                 ::VariantClear( &myArgValues[i] );
             }
         }

         void setParam( size_t i, std::wstring const& s )
         {
             _variant_t v( s.c_str() );
             myArgValues.at( (myArgValues.size() - 1) - i ) = v.Detach();
         }

         DISPPARAMS* ptr() { return &myParams; }
     };

     class OleAutomationObject
     {
     private:
         detail::IDispatchPtr myDispatch;

         enum CallTypeEnum
         {
             methodCall = DISPATCH_METHOD,
             propertyGet = DISPATCH_PROPERTYGET,
             propertyPut = DISPATCH_PROPERTYPUT,
             propertyPutRef = DISPATCH_PROPERTYPUTREF
         };

         _variant_t invokeAs(
             CallTypeEnum callType,
             wchar_t const name[],
             DispatchParams& params
             ) const
         {
             using namespace detail;
             EXCEPINFO exceptionInfo = {};
             _variant_t returnValue;
             unsigned erronousArgIndex = 0;

             DISPID const dispId = dispIdOf( name );
             HRESULT const hr = myDispatch->Invoke(
                 dispId,
                 IID_NULL, // reserved
                 0, // locale id
                 static_cast<WORD>( callType ),
                 params.ptr(),
                 &returnValue, // VARIANT FAR* pVarResult
                 &exceptionInfo, // EXCEPINFO FAR* pExcepInfo
                 &erronousArgIndex
                 );
             (SUCCEEDED( hr )) || throwHRX( "OleAutomationObject: Invoke: ", hr );
             return returnValue;
         }

     public:
         OleAutomationObject( wchar_t const progId[], DWORD serverType =
CLSCTX_ALL )
             : myDispatch( progId, 0, serverType )
         {
             using namespace detail;
             (myDispatch != 0) || throwX( "OleAutomationObject::<init> failed" );
         }

         DISPID dispIdOf( wchar_t const name[] ) const
         {
             using namespace detail;
             DISPID dispId = 0;

             HRESULT const hr = myDispatch->GetIDsOfNames(
                 IID_NULL, // reserved
                 const_cast<wchar_t**>( &name ),
                 1, // count of names
                 0, // locale id
                 &dispId
                 );
             (SUCCEEDED( hr )) ||
                 throwHRX( "OleAutomationObject::dispIdOf: GetIDsOfNames: ", hr );
             return dispId;
         }

         _variant_t call( wchar_t const methodName[], DispatchParams& params )
         {
             return invokeAs( methodCall, methodName, params );
         }

         _variant_t call( wchar_t const methodName[], int nArgs )
         {
             DispatchParams params( nArgs );
             return call( methodName, params );
         }

         _variant_t get( wchar_t const propertyName[] ) const
         {
             // E.g. WordApp.Name requires non-zero arg pointers. Silly.
             DispatchParams params( 0 );
             return invokeAs( propertyGet, propertyName, params );
         }

         _bstr_t getAsBString( wchar_t const propertyName[] ) const
         {
             return get( propertyName );
         }

         std::wstring getAsStdString( wchar_t const propertyName[] ) const
         {
             return getAsBString( propertyName ).operator wchar_t const*();
         }
     };
}

#endif
</code>

<code file="main.cpp">
#include <cassert>
#include <iostream>
#include <ostream>
#include <stdexcept>
#include <string>

#include "OleAutomationObject.hpp"

bool throwX( char const s[] ) { throw std::runtime_error( s ); }

class WshShell: public alfs::OleAutomationObject
{
private:
     void quit()
     {
         try
         {
             call( L"Quit", 3 ); // 3 defaulted arguments.
         }
         catch( std::exception const& )
         {
             assert( "Word application object failed to quit" && 0 );
             std::abort();
         }
     }

public:
     WshShell(): alfs::OleAutomationObject( L"WScript.Shell" ) {}

     long run( std::wstring const& path )
     {
         alfs::DispatchParams params( 3 );

         params.setParam( 0, path );
         return call( L"Run", params );
     }
};

struct ComUsage
{
     ComUsage()
     {
         (SUCCEEDED( CoInitialize(0) )) || throwX( "CoInitialize failed" );
     }

     ~ComUsage() { CoUninitialize(); }
};

void cppMain()
{
     ComUsage com;
     WshShell wshShell;

     std::cout << wshShell.run( L"notepad" ) << std::endl;
}

int main()
{
     try
     {
         cppMain();
         return EXIT_SUCCESS;
     }
     catch( std::exception const& x )
     {
         std::cerr << "!" << x.what() << std::endl;
         return EXIT_FAILURE;
     }
} // main
</code>

<output>
     <comment>Notepad appears, 0 as program's own output</comment>
</output>

Cheers, & hth.,

- Alf

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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