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Marshalling is used within implementations of different remote procedure call (RPC) mechanisms, where it is necessary for transporting data between processes and/or between threads. In Microsoft's COM, interface pointers must be marshalled when crossing COM apartment boundaries (that is, crossing between instances of the COM library). In the .NET Framework, the conversion between an unmanaged type and a CLR type, as in the P/Invoke process, is also an example of an action that requires marshalling to take place.
Additionally, marshalling is used extensively within scripts and applications that utilize the XPCOM technologies provided within the Mozilla Application Framework. The Mozilla Firefox browser is a popular application built with this framework that additionally allows scripting languages to utilize XPCOM through Cross-Platform Connect (XPConnect).
To "marshal" an object means to record its state and codebase(s) in such a way that when the marshalled object is "unmarshalled", a copy of the original object is obtained, possibly by automatically loading the class definitions of the object. You can marshal any object that is serializable or remote. Marshalling is like serialization, except marshalling also records codebases. Marshalling is different from serialization in that marshalling treats remote objects specially. (RFC 2713)
Note that 'codebase' here is used in its Java-specific meaning, to refer to a list of URLs where the object code can be loaded from, rather than in the more general meaning of codebase which refers to source code.
To "serialize" an object means to convert its state into a byte stream in such a way that the byte stream can be converted back into a copy of the object.