MSBuild: A Real-World Recursive Application

I recently posted on this blog a “toy application” of MSBuild that calculates factorials. Well, this weekend I was working on the new build script for the Nito.Async library, and surprised myself by finding an actual real-world application for this code!

It turns out that this is useful when autogenerating publisher policies. Nito.Async follows a simple major.minor version numbering scheme, where changes in minor are always fully backwards-compatible and changes in major never are. Publisher policies are a way of declaring backwards compatibility for strongly-named assemblies in the GAC (more info on MSDN and in KB891030).

To autogenerate publisher policies for a version maj.min, the build script must build a separate dll for each version in the range [maj.0, maj.min). It turns out that the recursive behavior in my “factorial.proj” toy was exactly what I needed; I just changed the return value to concatenate a list of numbers instead of multiplying them together.

There was one other small hurdle to overcome; I had to perform a cross product of two different item groups (the list of “previous minor versions” and the list of library dlls). This is not exactly straightforward in MSBuild, and is a common question (just Google for “MSBuild cross product”).