Mono.Cecil VS Fody

You mean Mono Cecil, native IL Emit or Reflection.Emit, or maybe LINQ Expressions?

Here's the documentation from a library in a project I'm working on right now: https://github.com/jbevain/cecil/wiki/

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.