COM Expertise needed: COM Callbacks

[John Chapman via Digitalmars-d-learn]

On Thursday, 27 April 2017 at 20:20:23 UTC, Nierjerson wrote:
>
> I think the main issue though, is that I really don't know what 
> is going on when I invoke the PS function. It seems to call the 
> server method that takes the interface and then the server does 
> it's "magic"(which is calling my QueryInterface) but how the 
> implemented QueryInterface is suppose to respond is beyond 
> me... I've tried some based stuff but nothing seem to work. The 
> good news is that it is doing something(calling QueryInterface) 
> which means that the server is at work.
>
> Any more ideas?  I think the issue currently is is the 
> QueryInterface(it is simply not doing what it is suppose to). 
> I'll probably have to look at some other implementations to see 
> what is going on.

QueryInterface is COM's version of opCast. It asks if you support 
the interface represented by an IID (riid). If you don't, then 
you return E_NOINTERFACE. If you do, then you point the result 
(pvObject) to yourself and return S_OK. Here's a basic 
implementation:

extern(Windows)
HRESULT QueryInterface(IID* riid, void** pvObject) {
   if (pvObject is null) return E_POINTER;
   *pvObject = null;

   if (*riid == IID_IUnknown) *pvObject = 
cast(void*)cast(IUnknown)this;
   else if (*riid == IID_IDispatch) *pvObject = 
cast(void*)cast(IDispatch)this;
   // and so on for all interfaces we support

   if (*pvObject is null) return E_NOINTERFACE;
   (cast(IUnknown)this).AddRef();
   return S_OK;
}

AddRef/Release perform the COM object's reference counting, so 
you should implement them too.

However, I don't understand why your icRBCColor class both 
implements and encapsulates IDispatch - the generated version 
cRGBColor from Gen.d just encapsulates it. Why do you need to 
instantiate an instance of icRGBColor? Can't you just use the 
rgb1 object you got from the dd.RGB() getter, assign the colour 
values to its Red, Blue, Green properties as needed, then call 
the dd.RGB(rgb1) setter? Does that not work?