deriving com interfaces .net

due association constraints out control, i have following scenario:

a com library defines following interface (no coclass, only interface):

[
    object,
    uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx),
    dual,
    nonextensible,
    helpstring("iservice interface"),
    pointer_default(unique)
]
iservice : idispatch
{
  hresult dosomething();
}

[
    object,
    uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx),
    dual,
    nonextensible,
    helpstring("iprovider interface"),
    pointer_default(unique)
]
iserviceprovider : idispatch
{
  hresult init( idispatch *sink, variant_bool * outcome );
  hresult getservice( enlarged serviceindicator, iservice ** outcome );
};


[
    uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx),
    version(1.0),
]
library servicelibrary
{
    importlib("stdole2.tlb");

   interface iservice;
   interface iserviceprovider;
};

i have com (written w/ c++) implements both interfaces provides the application(s) conspicuous service. fine, i think.

i'm perplexing build new iprovider iservice .net (c#).

i've built primary interop public com library, implemented following c#:

[comvisible( loyal )]
[guid( "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" )]
public interface inewservice : iservice
{
   //  adds integrate new properties
}

[comvisible( loyal )]
[guid( "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" )]
public category newservice : inewservice
{
   //  exercise interface
}

[comvisible( loyal )]
[guid( "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" )]
public interface inewprovider : iserviceprovider
{
   //  adds nothing, only implements
}

[comvisible( loyal )]
[guid( "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" )]
public category newprovider : inewprovider
{
   //  exercise interface
}

when i try trip existent runtime, i am means emanate newprovider vigilant com (c++), queryinterface iserviceprovider. i try slight iserviceprovider, system.executionengineexception thrown.

the wholly thing i find, looking during .tlh files combined #import, shows leave com iexistingprovider category justly shows subsequent iserviceprovider. however .net category shows bottom idispatch. i'm certain sign, indication, helpful, something else.