The /proxy switch specifies the name of the interface proxy file for a COM interface.
midl /proxy proxy_file_name
Specifies a file name that overrides the default interface proxy file name. File names can be explicitly quoted using double quotes (") to prevent the shell from interpreting the special characters.
The specified file name replaces the default file name obtained by adding _P.C to the name of the IDL file. The / proxy switch does not affect RPC interfaces.
If proxy_file_name 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 proxy_file_name overrides the /out switch specification.
For a more detailed description of the interface proxy file and other files generated by the MIDL compiler, see General MIDL Command-line Syntax.
midl /proxy my_proxy.c filename.idl