The /header switch specifies the name of the header file.
midl /header filename
Specifies a header file name that overrides the default header file name. File names can be explicitly quoted using double quotes (") to prevent the shell from interpreting special characters.
The specified file name replaces the default file name. The default file name is obtained by replacing the IDL file name extension (usually .idl) with the file name extension .h. For OLE interfaces, the /header switch overrides the default name of the interface header file.
When you are importing files, the specified file name applies to only one header file — the header file that corresponds to the IDL file specified on the command line.
If filename does not include an explicit path, the file is written to the current directory or the directory specified by the /out switch. An explicit path in filename overrides the /out switch specification.
midl /header "bar.h" filename.idl