Modding Tutorials/Writing custom code

In addition to creating data for the game to use, you can also write code. You could probably write in any .NET language, but I’ve only tested C#.

1. Create a new class library project in your code editor of choice. I use Visual Studio, but MonoDevelop also works well.
2. In your project, add references to these DLLs:
   

   (RimWorldInstallFolder)/RimWorld_Data/Managed/Assembly-CSharp.dll
   (RimWorldInstallFolder)/RimWorld_Data/Managed/UnityEngine.dll

3. In your project properties, change the target framework to .NET 3.5
4. Create a new class in a new code file.
5. You’ll want to add these namespace to each of your source files as necessary.
   

   using UnityEngine;  	//For all Unity functionality, rendering, resource management
   using AI;		//RimWorld AI
   using Sound;	        //RimWorld sound subsystem
   using UI;		//RimWorld GUI
   

6. Write your class. Use ILSpy (Download Binaries) to open Assembly-CSharp.dll, which decompiles the game's source code. Any modding questions can be asked on the subforum.
7. Compile your class into a .dll. Make sure your project's output type is "class library".
   Note that by default, Visual Studio will compile all the references of the project as well, so you’ll get a copy of UnityEngine.dll and Assembly-CSharp.dll and some others. You don’t need these. Just take YourModName.dll.
8. Place the .dll in the YourModeName/Assemblies folder of your mod.
9. Reference the classes in your .dll from the xml data in the YourModName/Defs folder. For example, you could create a new ThingDef with a <thingClass> that points to a class in your .dll.
10. The game should load your class now.
11. If you wish, you should also release your source code in the YourModName/Source directory.

You can find a small tutorial project here: Modding Tutorials/Assembly Modding Example