Fody VS ModuleInit

While Fody works at binary (MSIL) level, Metalama Compiler is a fork of Roslyn and allows for AST transformations, at the C# level.

As an alternative to PostSharp you may take a look at Fody.

Working with IL weaving as part of the building process is quite complex though, and this is where Fody enters into the picture. Fody is an extensible tool that allows to simplify the weaving process by taking care of all the heavy lifting.

Fodyplugins can modify exists code during compilation.

I've seen coworkers using Fody for such MSIL weaving stuff, but I have a bad feeling about this. It's a little bit like jailbreaking your phone. Cool stuff ahead, but voids warranty.

Implement your own (in C++/outside dotnet/CLR) IHostAssemblyManager stuff, this is the "replace/bypass how assemblies are loaded" trick. There used to be a prebuilt thing on the old old codepages, but I can't find it. I wonder if newer code/assemblie weavers such as Fody can do the similar thing?

Great place to start is https://github.com/Fody/Fody and existing 'addins' - fody does it at compile time - generally quite useful for your own code if you want to dip into aspect oriented programming.

Module initializers are a kinda obscure feature of .NET that allows writing initialization code for an assembly. They allow libraries to do one-time initialization when loaded, without the need for the user to explicitly call anything. It's a quite niche feature, but unfortunately it was not exposed in C#, and that led to users finding their own way of using it, like with ModuleInit.Fody. Given the need for it, Microsoft then decided to add it in supported platform starting from C# 9.0.