I'm trying to marshal an interface to another thread.

Windows provides the convenient CoMarshalInterThreadInterfaceInStream helper function to take care of the boilerplate code associated with using CoMarshalInterface directly.

const Guid CLSID_Widget = "{F8383852-FCD3-11d1-A6B9-006097DF5BD4}";
const Guid IID_IWidget  = "{EBBC7C04-315E-11D2-B62F-006097DF5BD4}";

//Create our widget
HRESULT hr = CoCreateInstance(CLSID_Widget, null, 
      CLSCTX_INPROC_SERVER | CLSCTX_LOCAL_SERVER, 
      IID_IWidget, out widget);
OleCheck(hr);

//Marshall the interface into an IStream
IStream stm;
hr = CoMarshalInterThreadInterfaceInStream(IID_IWidget, widget, out stm);
OleCheck(hr);

Except that the call to CoMarshalThreadInterfaceInStream fails with:

REGDB_E_IIDNOTREG (0x80040155)
Interface not registered

Go directly to CoMarshalInterface

The COM API function CoMarshalInterThreadInterfaceInStream provides a simple wrapper around CreateStreamOnHGlobal and CoMarshalInterface, as shown here:

// from OLE32.DLL (approx.)
HRESULT CoMarsha1InterThreadInterfaceInStream(
   REFIID riid, IUnknown *pItf, IStream **ppStm) 
{
   HRESULT hr = CreateStreamOnHGlobal(0, TRUE, ppStm);
   if (SUCCEEDED(hr))
       hr = CoMarshalInterface(*ppStm, riid, pItf,
             MSHCTX_INPROC, 0, MSHLFLAGS_NORMAL);
       return hr;
}

So we can try ourselves.

IStream stm = new Stream()
hr = CoMarshallInterface(stm, IID_IWidget, widget, 
      MSHCTX_INPROC,    // destination context is in-process/same host
      NULL,             // reserved, must be null
      MSHLFLAGS_NORMAL  // marshal once, unmarshal once
);
OleCheck(hr);

But that fails with:

REGDB_E_IIDNOTREG (0x80040155)
Interface not registered

Use standard marshaling

My class does not implement IMarhsal interface. This is right and normal.

By default, when CoMarshalInterface is first called on an object, the object is asked whether it wishes to handle its own cross-apartment communications. This question comes in the form of a QueryInterface request for the IMarshal interface. Most objects do not implement the IMarshal interface and fail this QueryInterface request, indicating that they are perfectly happy to let COM handle all communications via ORPC calls. Objects that do implement the IMarshal interface are indicating that ORPC is inappropriate and that the object implementor would prefer to handle all cross-apartment communications via a custom proxy. When an object implements the IMarshalinterface all references to the object will be custom marshaled.

When an object does not implement the IMarshal interface, all references to the object will be standard marshaled. Most objects elect to use standard marshaling.

So the the question becomes why is the standard COM marshaler having so much problems? What is the source of the error Interface not registered?

The interface is, in fact, not registered

The requires for COM are undocumented, but i can tell you that my interface GUID does not exist in:

HKEY_CLASSES_ROOT\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}

The reason for this will be explained at the end.

I know that Windows provides you the CoRegisterPSClsid function that allows you to register an interface inside your process, so that the standard marshaler will be able to marshal it:

Enables a downloaded DLL to register its custom interfaces within its running process so that the marshaling code will be able to marshal those interfaces.

HRESULT CoRegisterPSClsid(
   _In_ REFIID   riid,
   _In_ REFCLSID rclsid
);

Parameters:

• riid [in]: A pointer to the IID of the interface to be registered.
• rclsid [in]: A pointer to the CLSID of the DLL that contains the proxy/stub code for the custom interface specified by riid.

Which i can try calling, but what clsid do i use?

CoRegisterPSClsid(IID_IWidget, ???);

What is the CLSID of the DLL that contains the proxy/stub code for the custom interface specified by riid? Do i use my class itself?

CoRegisterPSClsid(IID_IWidget, CLSID_Widget);

That doesn't sound right; but i don't understand the COM standard marshaler well enough. Doesn't that CLSID have to be one of the standard COM marsharling classes; implementing IPSFactoryBuffer?

Either way, it doesn't work. I still get the error "Interface not registered".

Register the interface

I of course can register my interface in the registry:

HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}
      (default) = "IWidget"

But that doesn't fix it. Spelunking through the Interface registry keys, i notice that many specify a ProxyStubClsid32 entry.

When a new interface is requested on an object, the proxy and stub managers must resolve the requested IID onto the CLSID of the interface marshaler. Under Windows NT 5.0, the class store maintains these mappings in the NT directory, and they are cached at each host machine in the local registry. The machine-wide IID-to-CLSID mappings are cached at

HKEY_CLASSES_ROOT\Interface

and the per-user mappings are cached at

HKEY_CURRENT_USER\Software\Classes\Interface

One or both of these keys will contain a subkey for each known interface. If the interface has an interface marshaler installed, there will be an additional subkey (ProxyStubClsid32) that indicates the CLSID of the interface marshaler.

Except what class implements marshaling? I don't have a marshaler.

Can COM automatically marshal based on a TypeLibrary

Is it possible that if i register a Type Library with my Interface, that COM's standard marshaler will be able to bootstrap a proxy class on the fly?

I registered my interface above. Now i manually include the TypeLibrary:

HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}\TypeLib
      (default) = "{38D528BD-4948-4F28-8E5E-141A51090580}"

And if i monitor the registry during the call to CoMarshalInterface i see that it attempts, and does find, my Interface IID:

• Operation: RegOpenKey
• Path: HKCR\WOW6432Node\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}
• Result: SUCCESS

It then tries to look for a ProxyStubClsid32, and fails:

• Operation: RegOpenKey
• Path: HKCR\WOW6432Node\Interface\{668790E3-83CC-47E0-907F-A44BA9A99C8D}\ProxyStubClsid32
• Result: NAME NOT FOUND

My hope would then be that the standard COM marshaler attempts to look for:

• Operation: RegOpenKey
• Path: HKCR\WOW6432Node\Interface\{668790E3-83CC-47E0-907F-A44BA9A99C8D}\TypeLib

But it doesn't.

The OLE Automation marshaler

According to Don Box, the Ole Automation marshaler (PSOAInterface - {00020424-0000-0000-C000-000000000046}) is able to build a stub/proxy out of a type library:

The Type Library Marshaler

When these specially annotated interfaces are encountered by RegisterTypeLib (or LoadTypeLib in legacy mode), COM adds ProxyStubClsid32 entries for the interface with the value {00020424-0000-0000-C0000-000000000046}. This GUID corresponds to the class PSOAInterface that is registered as living in OLEAUT32.DLL, the OLE automation DLL. Because of the DLL that it lives in, this marshaler is sometimes called the [oleautomation] marshaler, although it is also called the type library marshaler or the universal marshaler. I'll refer to it as the type library marshaler, since it really has very little to do with IDispatch. (In fact, it is common to use the [oleautomation] attribute on interfaces that don't derive from IDispatch directly or indirectly.)

The class factory for the type library marshaler does something very tricky in its CreateProxy and CreateStub routines. Rather than return a statically compiled vtable (which is impossible given the fact that the requested interface didn't exist when OLEAUT32.DLL was built as part of the OS), the type library marshaler actually builds an /Oicf-style proxy and stub based on the type library for the interface. Because there is no efficient way to find the ITypeInfo for an arbitrary interface, the LIBID and version of the interface's type library must be stored under the:

HKCR\Interface\{XXX}\TypeLib

registry key.

I tried to set PSOAInterface as my interface's ProxyStub class:

• Register standard COM class PSOAInterface as our proxy stub clsid

  HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}\ProxyStubClsid32
         (default) = "{00020424-0000-0000-C0000-000000000046}"
  

• Register our type library for our interface

  HKEY_CURRENT_USER\Software\Classes\Interface\{EBBC7C04-315E-11D2-B62F-006097DF5BD4}\TypeLib
         (default) = "{38D528BD-4948-4F28-8E5E-141A51090580}"
         Version   = "1.0"
  

• The type library itself is already registered:

  HKEY_CURRENT_USER\Software\TypeLib\{38D528BD-4948-4F28-8E5E-141A51090580}\1.0\0\win32
         (default) = "D:\Junk\ComLibraryProxyTest\Win32\Project1.dll"
  

But it still fails.

• it reads HKCR\Interface\[IID_IWidget]
• it reads HKCR\Interface\[IID_IWidget]\ProxyStubClsid32

But it:

• never reads: HKCR\Interface\[IID_IWidget]\TypeLib

So it fails to proxy the object.

Questions

• Is it possible for the standard COM "Ole Automation" marshaler to build a proxy class out of a Type Library at runtime?
• Is it possible for me to build a proxy class out of a Type Library at runtime?

Background

CoMarshalInterface takes an interface pointer on input and writes the serialized representation of the pointer to a caller-provided byte stream. This byte stream can then be passed to another apartment, where the CoUnmarshalInterface API function uses the byte stream to return an interface pointer that is semantically equivalent to the original object yet can be legally accessed in the apartment that executes the CoUnmarshalInterface call. When calling CoMarshalInterface, the caller must indicate how far away the importing apartment is expected to be. COM defines an enumeration for expressing this distance:

enum MSHCTX {
    MSHCTX_INPROC = 4,           // in-process/same host
    MSHCTX_LOCAL = 0,            // out-of-process/same host
    MSHCTX_NOSHAREDMEM = 1,      //16/32 bit/same host
    MSHCTX_DIFFERENTMACHINE = 2, // off-host
};

It is legal to specify a greater distance than required, but it is more efficient to use the correct MSHCTX when possible. CoMarshalInterface also allows the caller to specify the marshaling semantics using the following marshal flags:

enum MSHLFLAGS {
    MSHLFLAGS_NORMAL,      // marshal once, unmarshal once
    MSHLFLAGS_TABLESTRONG, // marshal once, unmarshal many
    MSHLFLAGS_TABLEWEAK,   // marshal once, unmarshal many
    MSHLFlAGS_NOPING = 4   // suppress dist. garbage collection

Normal marshaling (sometimes called call marshaling) indicates that the marshaled object reference must be unmarshaled only once, and if additional proxies are needed, additional calls to CoMarshalInterface are required. Table marshaling indicates that the marshaled object reference may be unmarshaled zero or more times without requiring additional calls to CoMarshalInterface.

I think that all COM object type libraries, when compiled by MIDL, can automatically create a proxy/stub factory. But in my case:

if the COM standard marshaler can't find a proxy/stub factory for an interface, it returns the error REGDB_E_IIDNOTREG.

It may be the case that i have to either:

• use CreateProxyFromTypeInfo and CreateStubFromTypeInfo to create my own proxy
• let the standard COM marshaler automatically create a proxy/stub if there's a typeinfo chunk associated with the interface GUID.

Bonus Reading

• Old New Thing: What are the rules for CoMarshalInterThreadInterfaceInStream and CoGetInterfaceAndReleaseStream?
• Old New Thing: Why do I get the error REGDB_E_IIDNOTREG when I call a method that returns an interface?
• Old New Thing: Why do I get E_NOINTERFACE when creating an object that supports that interface?
• Don Box - MSJ: Standard Marshalling in COM (archive)
• Mysterious disappearing acts by samples from Microsoft SDKs

What CoMarshalInterface actually needs is IMarshal implementation (pointer) for the given interface, so that API could request the marshaling magic from it and, in particular, request IMarshal::GetUnmarshalClass in order to obtain information who is going to do the reverse magic afterwards:

This method is called indirectly, in a call to CoMarshalInterface, by whatever code in the server process is responsible for marshaling a pointer to an interface on an object. This marshaling code is usually a stub generated by COM for one of several interfaces that can marshal a pointer to an interface implemented on an entirely different object.

You don't have IMarshal implemented on your widget, so you are going to get it somewhere from.

As you started your question mentioning that you "want to marshal an interface to another thread" and the code comment says "Create our widget" there is a chance that you can utilize IMarhsal implementation of free threaded marshaler. The question does not provide information to tell whether it is possible and/or acceptable solution.

Back to the challenge of obtaining a marshaler, you are trying to work this around by utilizing a "standard" marshaler by "registering the interface":

why is the standard COM marshaler having so much problems

[...]

I of course can register my interface in the registry

Well, this is not how things actually work.

Interface registration is not just a registry key that "okay, this IID has its own key in the registry". The purpose of such registration is to point where to look for proxy/stub pair for this interface. Your manual creating of registry entries cannot help here if you don't have a proxy/stub DLL for the interface in question. If you had it, you would just regsvr32 it the usual way to create the registry keys.

So called standard marshaler, your next try, is not supposed to marshal any interface and your IWidget in particular. OLE supplies so called "PSOAInterface" marshaler, which is capable of supplying proxy/stub pairs for OLE Automation compatible interfaces. Just for them! The marshaler does not have so much problems, it actually has just one: your IWidget is unlikely to be compatible or you would not have the problem in first place.

If your IWidget was compatible, had an associated type library, where it was marked as [oleautomation], the type library registration process would have automatically created the registry keys referencing PSOAInterface and supplying ProxyStubClsid32. Then marshaling API would have picked PSOAInterface for your widget, it would have picked up the registered type library, load details of the interface, then provided standard proxy/stub pair for it and that's it. Standard marshaler works just within these contraints and not just for any interface pointed to.

That is, your options are:

• implement IMarshal on the widget server
• make IWidget OLE Automation compatible, with type library, so that type library registration process activates "standard marshaler" PSOAInterface for your interface
• build, if possible and applicable, proxy/stub implementation for your interface automatically generated by MIDL complier (you can check standard ATL DLL project, created from Visual Studio template on how it can be done - it creates additional project with "PS" suffix, for a supplementary proxy/stub DLL)
• implement separately IMarshal in a standalone DLL, and register it with the registry and your IWidget interface, so that marshaling API would pick it up when it attempts to marshal the interface pointer (I suppose it's the only option here if your IWidget is OLE incompatible and you have reasons to not alter the original implementation)
• use free threaded marshaler in the widget server if its restrictions are acceptable

Oh wait, hold on - there is another weird one. If you don't want or cannot afford, or you are reluctant to change the COM server (widget, that is) but you can modify client side code as you like, you can create a thin wrapper that implements two interfaces IWidget (all methods forward calls to real server) and IMarshal on client side, and pass its IWidget to the CoMarshalInterThreadInterfaceInStream API. This will force COM to use your marshaling without altering the original server. You are on your own, of course, to do the actual marshaling afterwards. It is unlikely that it matches your actual need, and it is just an abstract note to the discussion (which mostly consists of attempts to do impossible without details on interface itself and available options on modification of server implementation).