Custom Build Rules Your Visual C++ project may require a complicated build process in which some files need to be built by external tools. Now you can accomplish this more easily by using the new custom build rules in Visual C++ 2005. Custom build rules, which are defined in XML .rule files, facilitate building file extensions that require external build tools unknown to the Visual C++ project system. You can define custom build rules and incorporate them into the build process of a Visual C++ project. A custom build rule enables you pass input files with certain extensions to a tool you specify, which results in one or more output files, as part of the build process. Rule files follow the same property inheritance guidelines as property sheets (.vsprops files). You can validate your own rule files using the XML schema definition provides by Visual C++.