Modding Tutorials/Modifying defs

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This tutorial shows you several ways to modify existing Defs and alternatives to changing the XML format for them.

There are several ways to achieve this goal. The table below is the TL;DR version of this article. C# related operations are marked bold.

Method	Pros	Cons	When to use
Overwrite the Def	Braindead easy	So incompatible you gotta be braindead	When you don't care about the original, or anyone else
XPath change	Highly specific, highly compatible	Limited to XML-defined Defs	When you need to change a few XML values
Adding a (self-made) Comp	Very flexible, well-supported, highly-compatible	Does not work on every Def	When you want to add functionality
DefModExtension	Very simple, well-supported, highly-compatible	Static data	When you want to add fields/data
SubClassing	Fairly powerful, half the work is already done	Compatibility issues, not very flexible	When neither a Comp nor a DefModExtension works
Custom Def	Full control	Very specific to your mod	When the given Defs don't suffice for you

Requirements

• Understanding Defs helps.
• For the C# portions:
  • Decompiling source code is required.
  • This tutorial requires you to know how to write custom code.
  • Introduction to Def Classes is highly recommended.
    
    

Overwriting Defs

Main article: Modding_Tutorials/Compatibility_with_defs

If two mods share the same defName for a single type of Def, the last mod wins. If mod A adds a ResearchDef with defName Pemmican and mod B also adds a ResearchDef with defName Pemmican, the game will use mod B's Pemmican.

Pros

Braindead easy.

Cons

No compatibility.

When to use

Not.

XPath

Main article: Modding Tutorials/PatchOperations

XPath allows you to change specific values of Defs (multiple in a single operation, if you want) across mods with surgical precision.

Pros

Highly flexible, highly compatible.

Cons

Limited to Defs defined in XML (no meat, corpses or other generated Defs). More complex operations require more fiddly syntax.

When to use

All the time.

Adding a (self-made) Comp

Main article: Modding Tutorials/ThingComp

ThingComps are like little modules you can add to any ThingWithComp to give them added functionality.

Pros

Very flexible, well-supported and highly-compatible. There are many ready-made (example) Comps available that can be employed to do a wide variety of things.

Cons

Not suited for every type of functionality.

When to use

When you want to add functionality, non-static data or behaviour.

DefModExtensions

Main article: Modding Tutorials/DefModExtension

DefModExtensions can be seen as a way to add fields to Defs.

Pros

Very simple, very lightweight, highly compatible.

Cons

Static data only.

When to use

When you want to add (static) fields/data to Defs.

Subclassing

Main article: Modding_Tutorials/Def_classes

Inherit from a Def and explicitly tell RimWorld to use that specific Type.

Pros

They're you're own Type, so you can extend their functionality until your heart is content.

Cons

• Still bound to the base Def without access to its private methods
• Using them requires a lot of casting
• Only one custom class per Def
• They don't offer a lot of extra functionality over DefModExtensions or C# Extension methods

When to use

When neither a Comp nor a DefModExtension works.

Custom Def

Main article: Modding Tutorials/Def_classes

(Optionally) inherit from a Def and tell RimWorld to use your custom Def.

Pros

Fully your own Type, complete control. No incompatibility issues, because they're all yours.

Cons

Implementation from scratch.

When to use

If your Def is really unique to your mod or specific goal.

Other ways

Sometimes creative, sometimes hacky:

Checking for tags

Instead of using custom defClasses and comps you could also use tags. This is especially useful for lightweight features and simple compatibility.

Some tags are never used by a certain Thing such as ApparelTag on a Building. If a tag is never used it doesn't throw an error and therefore you could introduce as many useless tags as you want to a mod without the game complaining. When other mods check for these tags they can also do it without a problem. This way you could add tags of whatever name you want and let others check for this tag for compatibility.

Pros

Lightweight, easy.

Cons

Risk of potential side-effects. Kinda hacky.

Changing the class used by the Def

A lot of Defs have a field entry that specifies the C# class they are tied to. For instance, the GameConditionDef for the Eclipse has a conditionClass of GameCondition_Eclipse. Modders who wish to add some sparkles to the Eclipse could create a new GameCondition class called GameCondition_EclipseWithSparkles and use XPath to change the conditionClass to MyNameSpace.GameCondition_EclipseWithSparkles.

Pros

Pretty easy. Keeps most values of the original Def intact as well.

Cons

Subverts any Harmony patches on the original Class, so potential issues with compatibility.

Harmony patching

Main article: Modding Tutorials/Harmony

If you still want to use Harmony in the face of all these alternatives, you might have fallen for the Golden Hammer Design Pattern. But sure, go ahead.

See also

• Compatibility with defs explains compatibility from the XML side of things.