COM Expertise needed: COM Callbacks

[Nierjerson via Digitalmars-d-learn]

On Friday, 28 April 2017 at 09:25:31 UTC, John Chapman wrote:
> On Thursday, 27 April 2017 at 20:20:23 UTC, Nierjerson wrote:
>>
>
> 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;
> }
>

Your code works. I  had something similar but I think I was 
returning a null pointer on IID_Unknown or something similar. I 
now get a different error but it is a COM error rather than 
access violation. (error 80020009)