Using GPPG and GPLEX with Visual Studio

Here’s a quick note on a problem I ran into awhile back. I was using the GPPG and GPLEX parser tools as part of a Visual Studio project – the input files for these tools generate C# source files which are then compiled into the project. However, I noticed a problem with the recommended project setup (basically, setting up a MSBuild .targets file for GPPG and GPLEX source files, and then including that target file in the project). Changes that I made to the grammar would only take effect the second time I built the project. The samples and documentation for MPPG and MPLEX (earlier versions of GPPG and GPLEX) are silent on this issue. After inserting some debug code into my build targets, I determined that the files being output by the GPPG and GPLEX target handlers were correct, but the compiler was still using the older versions of the files. It seemed like it was using a cached copy of the old version of the grammar.

As it turns out, there is a bit of chicanery going on inside Visual Studio that results in this behavior. Visual Studio actually runs the C# compiler in-process, as an optimization to avoid process start overhead. This in-process compiler gets fouled up, however, when C# source files are generated as part of a build step – it seems to load all of the source files when the build starts, so it winds up using the old version instead of the freshly generated one.

The solution is to add the UseHostCompilerIfAvailable property to the .csproj project file, like this:

<PropertyGroup>
	<UseHostCompilerIfAvailable>False</UseHostCompilerIfAvailable>
</PropertyGroup>

This will force Visual Studio to use the out-of-process compiler, which will cause the correct version of the grammar to be built. Building the project will be a little slower, but it’s better than having to build twice!