Harmony A library for patching, replacing and decorating .NET and Mono methods during
 

It was mentioned by @Levis and it is a really good tool with src.

About

Harmony gives you an elegant and high level way to alter the functionality in applications written in C#. It works great in games and is well established in titles like 7 Days To Die, BattleTech, Besiege, Cities:Skylines, Kerbal Space Program, Oxygen Not Included, Ravenfield, Rimworld, Sheltered, Stardew Valley, Staxel, Subnautica, The Ultimate Nerd Game, Total Miner, Unturned, SCP: Secret Laboratory and many more.

It is also used in unit testing WPF controls and in many other areas.
How it works

If you develop in C# and your code is loaded as a module/plugin into a host application, you can use Harmony to alter the functionality of all the available assemblies of that application. Where other patch libraries simply allow you to replace the original method, Harmony goes one step further and gives you:

• A way to keep the original method intact
• Execute your code before and/or after the original method
• Modify the original with IL code processors
• Multiple Harmony patches co-exist and don't conflict with each other
• Works at runtime and does not touch any files

Harmony

Here are some other projects that are similar:

- dnlib: https://github.com/0xd4d/dnlib
- Mono.Cecil: https://github.com/jbevain/cecil
- Fody: https://github.com/Fody/Fody
- Aspect Injector: https://github.com/pamidur/aspect-injector
- NConcern: https://github.com/Puresharper/NConcern
- Puresharp: https://github.com/Puresharper/Puresharp (Newer version of NConcern.)