Using the __midl Predefined Constant
When the MIDL compiler processes the input IDL and ACF files, __midl is defined by default and is used for conditional compilation to attain consistency throughout the build. This phases out the use of define statements in the header files, such as MIDL_PASS, and replaces them with a consistent flag. The value of the __midl constant indicates the compiler version major.minor according to the formula major * 100 + minor; for example MIDL ver. 6.0.x has the __midl defined as 600.
If you choose, you can override this default by specifying the following on the command line: /U__midl. For more information, see /U.