midl /ms_union
midl /ms_union file.idl
The /ms_union switch controls the NDR alignment of nonencapsulated unions.
The MIDL compiler mirrors the behavior of the OSF-DCE IDL compiler for nonencapsulated unions. However, because earlier versions of the MIDL compiler did not do so, the /ms_union switch provides compatibility with interfaces built on previous versions of the MIDL compiler.
The ms_union feature can be used as a command line switch (/ms_union), an IDL interface attribute, or as an IDL type attribute.
General MIDL Command-line Syntax, IDL, ms_union