Marshalling

Marshalling is the process of packaging and unpackaging parameters so a remote procedure call can take place. Different parameter types are marshalled in different ways. For example, marshalling an integer parameter involves simply copying the value into the message buffer (although even in this simple case, there are issues such as byte ordering to deal with with cross-machine calls). Marshalling an array, however, is a more complex process. Array members are copied in a specific order so that the other side can reconstruct the array exactly. When a pointer is marshalled, the data that the pointer is pointing to is copied following rules and conventions for dealing with nested pointers in structures. Unique functions exist to handle the marshalling of each parameter type.

There are two types of OLE marshalling: standard and custom.

With standard marshalling, the proxies and stubs are system-wide resources for the interface, and they interact with the channel through a standard protocol. Standard marshalling can be used by both standard OLE interfaces and custom interfaces:

• In the case of most OLE interfaces, the proxies and stubs for standard marshalling are themselves in-process component objects which are loaded from a system-wide DLL provided by OLE, OLE32.DLL.

• In the case of custom interfaces, the proxies and stubs for standard marshalling are generated by the interface designer, typically with MIDL. These proxies and stubs are statically configured in the registry, so any potential client can use the custom interface across process boundaries. These proxies and stubs are loaded from a DLL that is located via the system registry using the interface ID (IID) for the custom interface they marshal.

Standard marshalling is the default method for the provision of marshalling code when passing interface pointers. However, because multiple objects may implement a single interface any number of ways, standard marshalling may not be sufficient for a particular implementation. Therefore, any object may override the default and provide custom marshalling for its interfaces. Other implementations of the interface are not affected.

As an alternative to standard marshalling, an interface (standard or custom) can use custom marshalling. With custom marshalling, an object dynamically implements the proxies at run-time for each interface that it supports. For any given interface, the object can select OLE-provided standard marshalling or custom marshalling. This choice is made by the object on an interface by interface basis. Once the choice is made for a given interface, it remains in effect during the object's lifetime. However one interface on an object can use custom marshalling while another uses standard marshalling.

Custom marshalling is inherently unique to the object application that implements it. It uses proxies implemented by the object and provided to the system on request at run-time. Objects that custom-marshal must implement the IMarshal interface, whereas objects that support standard marshalling do not. For complete documentation of the IMarshal interface, see the OLE Reference Guide. Also, see the chapter "The Component Object Model" for more information on marshalling.

Marshalling support is provided by the system for all OLE-defined interfaces. This support is via component objects in the file OLE32.DLL. If you decide to write a custom interface, you must provide marshalling support for it. Typically, you will provide a standard marshalling DLL for the interface you design. You can use the tools contained in the Win32 SDK CD to create the proxy/stub code and the proxy/stub DLL.

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