CoMarshalInterface function (combaseapi.h)

Article
10/13/2021

Writes into a stream the data required to initialize a proxy object in some client process.

Syntax

HRESULT CoMarshalInterface(
  [in]           LPSTREAM  pStm,
  [in]           REFIID    riid,
  [in]           LPUNKNOWN pUnk,
  [in]           DWORD     dwDestContext,
  [in, optional] LPVOID    pvDestContext,
  [in]           DWORD     mshlflags
);

Parameters

[in] pStm

A pointer to the stream to be used during marshaling. See IStream.

[in] riid

A reference to the identifier of the interface to be marshaled. This interface must be derived from the IUnknown interface.

[in] pUnk

A pointer to the interface to be marshaled. This interface must be derived from the IUnknown interface.

[in] dwDestContext

The destination context where the specified interface is to be unmarshaled. The possible values come from the enumeration MSHCTX. Currently, unmarshaling can occur in another apartment of the current process (MSHCTX_INPROC), in another process on the same computer as the current process (MSHCTX_LOCAL), or in a process on a different computer (MSHCTX_DIFFERENTMACHINE).

[in, optional] pvDestContext

This parameter is reserved and must be NULL.

[in] mshlflags

The flags that specify whether the data to be marshaled is to be transmitted back to the client process (the typical case) or written to a global table, where it can be retrieved by multiple clients. The possible values come from the MSHLFLAGS enumeration.

Return value

This function can return the standard return values E_FAIL, E_OUTOFMEMORY, and E_UNEXPECTED, the stream-access error values returned by IStream, as well as the following values.

Return code	Description
S_OK	The HRESULT was marshaled successfully.
CO_E_NOTINITIALIZED	The CoInitialize or OleInitialize function was not called on the current thread before this function was called.

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:

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.
Gets the CLSID of the object's proxy by calling IMarshal::GetUnmarshalClass, using whichever IMarshal interface pointer has been returned.
Writes the CLSID of the proxy to the stream to be used for marshaling.
Marshals the interface pointer by calling IMarshal::MarshalInterface.

The COM library in the client process calls the CoUnmarshalInterface function to extract the data and initialize the proxy. Before calling CoUnmarshalInterface, seek back to the original position in the stream.

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.

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps \| UWP apps]
Minimum supported server	Windows 2000 Server [desktop apps \| UWP apps]
Target Platform	Windows
Header	combaseapi.h (include Objbase.h)
Library	Ole32.lib
DLL	Ole32.dll