In Delphi 12.3, this compiles and works to create a new empty Access database file:

Uses ComObj;
    
procedure TForm1.Button1Click(Sender: TObject);
var
  AdoX: OleVariant;    
begin
  AdoX := CreateOleObject('ADOX.Catalog');
  AdoX.Create('Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb');
end;

But I am struggling to do the same in C++Builder 12.3.

I started with the following:

#include <ComObj.hpp>
    
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    OleVariant AdoX;
    
    AdoX = CreateOleObject(L"ADOX.Catalog");
    AdoX.Create(L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb");
}

However, the Windows 64-bit (Modern) compiler gives this error:

[bcc64x Error] Unit1.cpp(29): use of overloaded operator '=' is ambiguous (with operand types 'System::OleVariant' and '_di_IDispatch' (aka 'DelphiInterface< ::IDispatch>'))

If I use _di_IDispatch like so:

#include <ComObj.hpp>
    
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    _di_IDispatch AdoX;
    
    AdoX = CreateOleObject(L"ADOX.Catalog");
    AdoX.Create(L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb");
}

Then the line with CreateOleObject() compiles, but the next line doesn't compile because _di_IDispatch has no Create() method:

[bcc64x Error] Unit1.cpp(30): no member named 'Create' in   System::DelphiInterface<IDispatch>'.

In Delphi 12.3, this compiles and works to create a new empty Access database file:

Uses ComObj;
    
procedure TForm1.Button1Click(Sender: TObject);
var
  AdoX: OleVariant;    
begin
  AdoX := CreateOleObject('ADOX.Catalog');
  AdoX.Create('Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb');
end;

But I am struggling to do the same in C++Builder 12.3.

I started with the following:

#include <ComObj.hpp>
    
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    OleVariant AdoX;
    
    AdoX = CreateOleObject(L"ADOX.Catalog");
    AdoX.Create(L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb");
}

However, the Windows 64-bit (Modern) compiler gives this error:

[bcc64x Error] Unit1.cpp(29): use of overloaded operator '=' is ambiguous (with operand types 'System::OleVariant' and '_di_IDispatch' (aka 'DelphiInterface< ::IDispatch>'))

If I use _di_IDispatch like so:

#include <ComObj.hpp>
    
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    _di_IDispatch AdoX;
    
    AdoX = CreateOleObject(L"ADOX.Catalog");
    AdoX.Create(L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb");
}

Then the line with CreateOleObject() compiles, but the next line doesn't compile because _di_IDispatch has no Create() method:

[bcc64x Error] Unit1.cpp(30): no member named 'Create' in   System::DelphiInterface<IDispatch>'.

• c++builder
• adox
• createoleobject

2 Answers 2

In Delphi, (Ole)Variant supports automatic method dispatching, meaning that when you call AdoX.Create(), the compiler recognizes that AdoX is an OleVariant and generates code which silently accesses the variant's IDispatch member and calls IDispatch.GetIDsOfNames() and IDispatch.Invoke() on it to invoke the requested COM object method dynamically at runtime, eg:

Uses ComObj;
    
procedure TForm1.Button1Click(Sender: TObject);
var
  AdoX: OleVariant;    
begin
  AdoX := CreateOleObject('ADOX.Catalog');

  AdoX.Create('Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb');
  //
  // the compiler really does something equivalent
  // to the following for you behind the scenes...
  //
  // var disp: IDispatch := IDispatch(AdoX);
  //
  // var name: PWideChar := 'Create';
  // var id: DISPID:
  // OleCheck(disp.GetIDsOfNames(IID_NULL, @name, 1, 0, @id));
  //
  // var wArgVal: WideString := 'Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb';
  // var vArg: VARIANTARG;
  // vArg.vt := VT_BSTR;
  // vArg.bstrVal := PWideChar(wArgVal);
  // var params: DISPPARAMS;
  // params.rgvarg := @vArg;
  // params.rgdispidNamedArgs = nil;
  // params.cArgs := 1;
  // params.cNamedArgs := 0;
  // var vResult: Variant;
  // var uArgErr: UINT;
  // var excepInfo: EXCEPINFO;
  // OleCheck(disp.Invoke(id, IID_NULL, 0, DISPATCH_METHOD, @params, @vResult, @excepInfo, @uArgErr));
  // raises an exception if excepInfo and/or uArgErr report an error...
  //
end;

But in C++, (Ole)Variant does not have that automatic dispatch capability, so you have to be a little more explicit about invoking COM object methods. In particular, when using _di_IDisptch, you need to use the -> operator to call methods on the IDispatch object that it holds, eg:

#include <ComObj.hpp>
    
void __fastcall TfrmRackmanDBMain::Button1Click(TObject *Sender)
{
    _di_IDispatch AdoX = CreateOleObject(L"ADOX.Catalog");
    AdoX->GetIDsOfNames(...);
    AdoX->Invoke(...);
}

Fortunately, you don't need to do that in this situation, because you can instead use the OleVariant::OleProcedure() method ¹ to do the hard work for you, eg:

#include <ComObj.hpp>
    
void __fastcall TfrmRackmanDBMain::Button1Click(TObject *Sender)
{
    OleVariant AdoX = CreateOleObject(L"ADOX.Catalog");
    AdoX.OleProcedure(L"Create", WideString(L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb"));
}

¹: use OleProcedure() when the COM method does not return a value. Use OleFunction() when it does.

That being said, you should consider using C++Builder's built-in ADO components, such as TADOConnection, etc instead of using ADO's COM API directly.

Thanks Remy - your answers are always very instructive.

The following code compiles and runs in C++Builder 12.3 64-bit (Modern):

void __fastcall TForm1::Button1Click(TObject *Sender)
{
    _di_IDispatch disp;

  disp = CreateOleObject(L"ADOX.Catalog");
  // AdoX.Create(L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb");

    //   the (Delphi) compiler really does something equivalent
    //   to the following for you behind the scenes...

    //  _di_IDispatch disp = _di_IDispatch(AdoX);

  PWideChar name = L"Create";
  DISPID id;
  OleCheck(disp->GetIDsOfNames(IID_NULL, &name, 1, 0, &id));

  WideString wArgVal = L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb";

  VARIANTARG vArg;
  vArg.vt = VT_BSTR;
  vArg.bstrVal = PWideChar(wArgVal);

  DISPPARAMS params;
  params.rgvarg = &vArg;
  params.rgdispidNamedArgs = nullptr;
  params.cArgs = 1;
  params.cNamedArgs = 0;
  VARIANT vResult;
  UINT uArgErr;
  EXCEPINFO excepInfo;

  OleCheck(disp->Invoke(id, IID_NULL, 0, DISPATCH_METHOD, &params, &vResult, &excepInfo, &uArgErr));
  // raises an exception if excepInfo and/or uArgErr report an error...
}

It does however keep the newly created Access database file locked (can't be deleted) until the application exits. It also creates the 'backup' Access database file 'ltest.accdb'.

The Delphi version doesn't do this. Neither does the OleProcedure method. Both of these methods only create (leave?) the main database file (test.accdb) and they both release it immediately. i.e., the file can be deleted and I assume connected to and used before the application exits.

void __fastcall TForm1::Button2Click(TObject *Sender)
{
  Variant AdoX = CreateOleObject(L"ADOX.Catalog");
  AdoX.OleProcedure(L"Create", WideString(L"Provider=Microsoft.ACE.OLEDB.12.0; Data Source=test.accdb"));
}

If you can shed any light on how a new access database can be created using TADOConnection that would be most appreciated.

I have tried to import the type library for ADOX so that I can use the ADOX.Catalog component. I was able to do this in the 32-bit IDE however I got class not registered. I put this down to a 32 / 64 bit mismatch.

In the 64-bit IDE, which I am working in here, the Import Component.. option available yet.