Reading a GUID Property from COM object with C++Builder

Question

I have a 3rd party COM object that I'm working with. Mostly fine, but I'm stuck on reading a GUID property from the object.

The relevant part of the auto-generated component wrappers/headers looks like this:

// *********************************************************************//
// DispIntf:  IFoo
// Flags:     (4416) Dual OleAutomation Dispatchable
// GUID:      {5DE5DAAF-5419-4B2B-9771-58EAEE780799}
// *********************************************************************//
template<class T>
class IFooDispT : public TAutoDriver<IFoo>
{
  ...
BSTR    __fastcall get_FileName(void);
HRESULT __fastcall get_ProjectGUID(/*AUTO_PARAM_ERROR(System::TGUID* Value)*/ VARIANT*  Value);
HRESULT __fastcall get_ProjectName(BSTR* Value/*[out,retval]*/);

__property   BSTR  FileName = {read = get_FileName};
__property   BSTR  ProjectName = {read = get_ProjectName};

Note how the ProjectGUID property is marked AUTO_PARAM_ERROR, and it doesn't appear in list of properties.

I've tried to read it directly via get_ProjectGUID() but it always returns HRESULT = 0x80070057 (E_INVALID_ARGS).

The IDL of the dispinterface from OleView looks like this:-

[
  uuid(5DE5DAAF-5419-4B2B-9771-58EAEE780799),
  version(1.0),
  helpstring("Dispatch interface for xpCOMFoo Object"),
  dual
]
dispinterface IFoo {
    properties:
    methods:
 <...snipped...>
        [id(0x000000cf), propget]
        BSTR FileName();
        [id(0x000000d0), propget]
        GUID ProjectGUID();
        [id(0x000000d1), propget]
        BSTR ProjectName();
};

I've tested the same object from Delphi (although, not using the late bindings shown above) and I'm happy that the COM object itself is not at fault.

Can you show the relevant part of the TLB's IDL with OleView? — noseratio
The COM object itself is at fault. Methods of a dispinterface need to be callable with IDispatch::Invoke(). Which requires that the method's argument and return values can be stored in a VARIANT. You can't store a GUID in a VARIANT. It is not actually a dispinterface, it has the [dual] attribute. You can only call the method early-bound. — Hans Passant

noseratio noseratio · Accepted Answer · 2014-04-27T01:57:14

Try calling it like this:

TAutoDriver<IFoo> foo;

// ...

GUID guid;
memset(&guid, 0, sizeof(guid));
// use -> to access the raw dual interface
HRESULT hr = foo->get_ProjectGUID(&guid);

Reading a GUID Property from COM object with C++Builder

1 Answers