Table of Contents

Build your first mod

This walkthrough starts with an empty folder and ends with a mod that prints the active Schedule I backend to the MelonLoader console. It is deliberately small. Get this working before adding game types, patches, or another modding library.

Before you start

You need:

  • Windows and PowerShell;
  • the .NET 8 SDK or newer;
  • the S1Interop repository;
  • a Schedule I Mono install with MelonLoader. The Steam alternate and alternate-beta branches use Mono.

Run this first:

dotnet --version
s1interop --version

The first command should print 8.0 or a newer major version. The second should print an S1Interop version. If s1interop is not found, follow Install S1Interop.

Note

The normal one-DLL build uses the Mono game files as compile-time references. You only need a separate IL2CPP install when you are ready to run the optional IL2CPP reference check or test the mod in that branch.

1. Preview the project

From the S1Interop repository, ask the CLI what it would create:

s1interop new ..\MyFirstMod --dry-run

The output should start with S1Interop new project dry-run: MyFirstMod and list a solution, project file, ModCore.cs, an S1Interop declarations file, and local setup files. Nothing is written during a dry run.

2. Create the project

Run the same command with --apply:

s1interop new ..\MyFirstMod --apply

S1Interop creates MyFirstMod beside the repository. It refuses to write into a non-empty target directory, which helps prevent an accidental overwrite when using the CLI.

3. Add your local paths

Copy the example props file:

Copy-Item ..\MyFirstMod\local.build.props.example ..\MyFirstMod\local.build.props

Open local.build.props and replace the placeholder values:

<Project>
  <PropertyGroup>
    <MonoGamePath>D:\Games\Schedule I_alternate</MonoGamePath>
    <Il2CppGamePath>D:\Games\Schedule I_public</Il2CppGamePath>
    <S1InteropGeneratorPackageSource>C:\Code\S1Interop\artifacts\packages</S1InteropGeneratorPackageSource>
    <RestoreAdditionalProjectSources Condition="'$(S1InteropGeneratorPackageSource)'!=''">$(S1InteropGeneratorPackageSource);$(RestoreAdditionalProjectSources)</RestoreAdditionalProjectSources>
  </PropertyGroup>
</Project>

Use the folder that contains Schedule I.exe for each game path. S1InteropGeneratorPackageSource must point to the artifacts\packages folder created when you installed the local alpha packages.

Only MonoGamePath and the package source are required for the first build. Leave Il2CppGamePath as a placeholder until you have a separate public or beta install with MelonLoader-generated assemblies.

local.build.props is ignored by the scaffold. Do not commit it, game DLLs, or generated IL2CPP assemblies.

4. Add the first bit of code

Replace MyFirstMod\ModCore.cs with this:

using MelonLoader;
using S1Interop.Generated;

[assembly: MelonInfo(typeof(MyFirstMod.ModCore), "MyFirstMod", "0.1.0", "YourName")]
[assembly: MelonGame("TVGS", "Schedule I")]

namespace MyFirstMod;

public sealed class ModCore : MelonMod
{
    public override void OnInitializeMelon()
    {
        LoggerInstance.Msg($"MyFirstMod loaded on {S1InteropRuntime.Backend}.");
    }
}

There are three pieces worth knowing:

  • MelonInfo tells MelonLoader which class starts the mod.
  • MelonGame limits the mod to Schedule I.
  • OnInitializeMelon runs when MelonLoader initializes the mod. The generated S1InteropRuntime helper reports whether the current game uses Mono or IL2CPP.

You do not need to understand generated facades yet. The generator package adds S1InteropRuntime during the build.

5. Build the DLL

Move into the new project and build it:

Set-Location ..\MyFirstMod
dotnet build .\MyFirstMod.sln -c Debug

A successful build ends with Build succeeded. The DLL is written to:

bin\Single\Debug\netstandard2.1\MyFirstMod.dll

If the build reports a missing MelonLoader or game assembly, check that MonoGamePath points to the game folder, not Schedule I_Data or one of its subfolders. See Common issues for the exact fixes.

6. Run it in Schedule I

Copy MyFirstMod.dll into the game's Mods folder, then launch Schedule I.

Look for this part of the message in the MelonLoader console or MelonLoader\Latest.log:

MyFirstMod loaded on Mono.

The same DLL should print MyFirstMod loaded on Il2Cpp. when loaded by an IL2CPP install. Unknown means the runtime probes did not find the expected Schedule I assemblies; keep the log and check Runtime shows Unknown.

What you have now

You built a normal MelonLoader mod. S1Interop supplied a compile-time generator and a small runtime helper inside your DLL; it did not add a separate runtime dependency.

Next, use Common tasks to add one game type, run an IL2CPP reference check, or analyze an existing mod. Read Core concepts when a term such as facade, declaration, or backend becomes relevant.

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