The /iid switch specifies the name of the interface identifier file for a COM interface, overriding the default name obtained by adding _i.c to the IDL file name.
midl /iid filename
Specifies an interface identifier file name that overrides the default interface identifier file name for a COM interface. File names can be explicitly quoted using double quotes (") to prevent the shell from interpreting the special characters.
The /iid switch does not affect RPC interfaces.
If filename does not include an explicit path, the file is written to the current directory or to the directory specified by the /out switch. An explicit path in filename overrides the /out switch specification.
midl /iid "my_iid.c" filename.idl