# CoMarshalInterface

Writes into a stream the data required to initialize a proxy object in some client process. The COM library in the client process calls the CoUnmarshalInterface function to extract the data and initialize the proxy. CoMarshalInterface can marshal only interfaces derived from IUnknown.

STDAPI CoMarshalInterface(

IStream *pStm, | //Pointer to the stream used for marshaling
REFIID riid, | //Reference to the identifier of the interface
IUnknown *pUnk, | //Pointer to the interface to be marshaled
DWORD dwDestContext, | //Destination context
void *pvDestContext, | //Reserved for future use
DWORD mshlflags | //Reason for marshaling
); |

Parameters

pStm
  [in] Pointer to the stream to be used during marshaling.
riid
  [in] Reference to the identifier of the interface to be marshaled. This interface must be derived from the IUnknown interface.
pUnk
  [in] Pointer to the interface to be marshaled; can be NULL if the caller does not have a pointer to the desired interface. This interface must be derived from the IUnknown interface.
dwDestContext
  [in] Destination context where the specified interface is to be unmarshaled. Values for dwDestContext come from the enumeration MSHCTX. Currently, unmarshaling can occur either in another apartment of the current process (MSHCTX_INPROC) or in another process on the same computer as the current process (MSHCTX_LOCAL).
pvDestContext
  [in] Reserved for future use; must be NULL.
mshlflags
  [in] Flag specifying whether the data to be marshaled is to be transmitted back to the client process&emdash;the normal case&emdash;or written to a global table, where it can be retrieved by multiple clients. Values come from the MSHLFLAGS enumeration.

Return Values

This function supports the standard return values E_FAIL, E_OUTOFMEMORY, and E_UNEXPECTED, as well as the following:

S_OK
  The interface pointer was marshaled successfully.
CO_E_NOTINITIALIZED
  The CoInitialize or OleInitialize function was not called on the current thread before this function was called.
IStream errors
  This function can also return any of the stream-access error values returned by the IStream interface.

Remarks

The CoMarshalInterface function marshals the interface referred to by riid on the object whose IUnknown implementation is pointed to by pUnk. To do so, the CoMarshalInterface function performs the following tasks:

1. Queries the object for a pointer to the IMarshal interface. If the object does not implement IMarshal, meaning that it relies on COM to provide marshaling support, CoMarshalInterface gets a pointer to COM’s default implementation of IMarshal.

2. Gets the CLSID of the object’s proxy by calling IMarshal::GetUnmarshalClass, using whichever IMarshal interface pointer has been returned.

3. Writes the CLSID of the proxy to the stream to be used for marshaling.

4. Marshals the interface pointer by calling IMarshal::MarshalInterface.

If you are implementing existing COM interfaces or defining your own interfaces using the Microsoft Interface Definition Language (MIDL), the MIDL-generated proxies and stubs call CoMarshalInterface for you. If you are writing your own proxies and stubs, your proxy code and stub code should each call CoMarshalInterface to correctly marshal interface pointers. Calling IMarshal directly from your proxy and stub code is not recommended.

If you are writing your own implementation of IMarshal, and your proxy needs access to a private object, you can include an interface pointer to that object as part of the data you write to the stream. In such situations, if you want to use COM’s default marshaling implementation when passing the interface pointer, you can call CoMarshalInterface on the object to do so.

See Also

CoUnmarshalInterface, IMarshal::MarshalInterface