Fody is a Nuget Packaged that allows you to decorate your code with attributes and from them generate code automatically through IL injection.
From the official project:
Manipulating the IL of an assembly as part of a build requires a significant amount of plumbing code. This plumbing code involves knowledge of both the MSBuild and Visual Studio APIs. Fody attempts to eliminate that plumbing code through an extensible add-in model.
It is also the name of this cute little bird 🙂