Question

I’m writing a C++ application which imports a COM DLL as following,

#import "MyLib.dll" no_namespace, raw_interfaces_only

There is a problem using method ‘_GetObject’ which is declared in the idl file like this,

[
  object,
  uuid(f022c0e0-1234-5678-abcd-c17d63954f4b),
  dual,
  nonextensible,
  helpstring("IStorageProxy Interface"),
  pointer_default(unique)
]
interface IStorageProxy : IDispatch
{
    [hidden, helpstring("method _GetObject")]
    HRESULT _GetObject(
            [in] BSTR entryId,
            [in] REFCLSID rclsid,
            [in] REFIID riid,
            [out, iid_is(riid), retval] IUnknown** stgObject);
};

But the generated tlh file has changed the types of the second and third parameters.

struct __declspec(uuid("f022c0e0-1234-5678-abcd-c17d63954f4b"))
IStorageProxy : IDispatch
{
  //
  // Raw methods provided by interface
  //

  virtual HRESULT __stdcall _GetObject (
    /*[in]*/ BSTR entryId,
    /*[in]*/ GUID * rclsid,
    /*[in]*/ GUID * riid,
    /*[out,retval]*/ IUnknown * * stgObject ) = 0;
};

As I’m coding against the original function signature (defined in the idl), so now the C++ code can’t compile. I’m not sure why the types changed to ‘GUID *’. Is there any way to stop the compiler from doing this?

Answer 1

No, that’s normal. Both REFGUID and REFIID are just a typedef for GUID*. Same kind of idea as a HWND and HDC, typedefs for HANDLE. These typedefs catch mistakes in C++ code, they are not really appropriate in a type library that supplies typeinfo to many languages.

You could technically keep these typedefs but those types will then have to be defined in the type library as well so that the COM client knows what they mean. The component author would have to include WTypes.idl. That’s probably too late by now, you can’t do anything about it in a COM client.