Problems reading embedded resources using system.reflection.metadata

As part of building a feature in ASP.NET Core I was using System.Reflection.Metadata to crack an assembly and extract an embedded resource to validate its contents (in a test).

I was using a snippet from https://github.com/dotnet/corefx/issues/23372 to do so, and I kept seeing a test fail intermittently on debug build on OS X in our CI build.

It seemed like sometimes it extracted the resource correctly and some other times it produced garbage. I was able to capture one of the generated assemblies that caused a test failure and I was able to produce a minimal repro (see below).

resourceextractor.zip

Upon debugging the repro a bit, I could see that the code used to calculate the offset to the resources was not correct and it was pointing past the resources directory. I can think one of 3 things are happening:

• There is a bug in peReader.TryGetDirectoryOffset.
• The generated assembly on Mac OS is somehow malformed.**
• The code snippet I used is not correct in all cases. (I can't tell for sure because it's the first time I use the API).

Any ideas?