As part of its instrumentation, NCrunch needs to load and dissect the built assembly in memory in order to manipulate it. On large assemblies, this can be a memory intensive process.

Is this project being included in your solution as a project dependency, or as a referenced DLL?

To ensure compatibility with all environments, NCrunch always uses an x86 process for building and instrumenting projects. I'll make a note to test the reliability of doing this under x64, as it may perform better this way anyway.

A single assembly file with 2M lines of code is fairly unconventional, so I think it's quite a safe bet that the sheer size of the project is causing the memory problems. The best workaround I can suggest is basically what you've already done - setting the 'Instrument output assembly' setting to 'False' will prevent NCrunch from processing the assembly for code coverage. As you've mentioned that this is a 3rd party component, my assumption is that you probably don't spend the bulk of your time working in this code and the code coverage is probably more or a nice to have.

I also hold a concern that the size of this project will likely cause performance problems for NCrunch even if we are able to solve the out of memory problems. If you have any opportunity to split up the project, I would definitely recommend doing so.

Meanwhile, I've made a note in the backlog to investigate adding an x86/x64 configuration option for the build processes.

Good thinking :) I'll look into it!