In this tutorial you will learn how to make a mod that runs on startup and writes "Hello World" to the dev console.

Requirements

This tutorial requires you to have set up a solution.
This tutorial requires a Mod folder (here called: MyMod) and the mod folder will need an About folder containing a valid About.xml and an Assemblies folder where we will put our HelloWorld.dll file. For more info, see Mod folder structure

Bootstrapping

There are two acceptable ways of making RimWorld load your mod. One is by using the StaticConstructorOnStartup annotation, the other is by inheriting from the Mod class. We will be using the StaticConstructorOnStartup annotation, for reasons explained below.

Hello World

using RimWorld;
using Verse;

namespace TestMod
{
    [StaticConstructorOnStartup]
    public static class MyMod
    {
        static MyMod() //our constructor
        {
            Log.Message("Hello World!"); //Outputs "Hello World!" to the dev console.
        }
    }
}

The [StaticConstructorOnStartup] annotation does what it says on the tin. At startup, it runs the Static Constructor of any class with that annotation. For a thorough explanation of what a static constructor is, refer to official C# documentation. What you need to know: It's a method without a return type, has the same name as the class and doesn't take any arguments.

Common pitfalls

Your constructor isn't static.
Your constructor isn't a constructor. Refer once more to the official C# documentation.
There are red squiggles under [StaticConstructorOnStartup] (or anywhere else): you either typo'd, are missing using statements or don't have a reference to Assembly-Csharp.dll. Read how to set up a solution again.
You did not compile, and if you did compile, your IDE placed the resulting assembly in some far away folder. Set the output path of your build to something more sane. Read how to set up a solution again.
There are lot of yellow errors along the line of "x already has short hash": You did not set "Copy local" to false. Read how to set up a solution again.

StaticConstructorOnStartup vs inheriting from Mod

Subclasses of the Mod class are loaded extremely early during launch. As such, it is less suitable for most purposes (most notably: using Defs or anything that uses Defs). Classes with the StaticConstructorOnStartup are initialised just before the Main Menu is shown. Classes with this annotation are loaded in the main thread: a requirement for loading any Texture, Material, Shader, Graphic, GameObject or MaterialPropertyBlock.

Inheriting from Mod is necessary if you want Mod Settings, or if you need to run really early in the loading process.

For the exact order of initialisation, look up Verse.PlayDataLoader.DoPlayLoad.

But why can't I make my StaticConstructorOnStartup inherit from mod?

That's like wearing two condoms. You might think you're extra safe, in reality it puts you in a worse position.

When C# tries to instantiate a type (like your Mod class) it first runs the static constructor and then an instance constructor. The static constructor in this case is the StaticConstructorOnStartup: meaning it'll try to load your textures outside the main thread and run your Harmony patches using Defs the game doesn't know exist yet.

Sometimes you need both a StaticConstructorOnStartup and a class that inherits from mod. The correct way of doing that is by making two separate classes; one that inherits from Mod and the other with a StaticConstructorOnStartup attribute.

Modding Tutorials/Hello World

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