MonoMod

General purpose .NET assembly modding "basework", powered by cecil.
^{MIT-licensed.}

GitHub: All	NuGet: Patcher	NuGet: Utils	NuGet: RuntimeDetour	NuGet: HookGen

^{... or download fresh build artifacts for the last commit.}

Sections

Special thanks to my patrons on Patreon:

Introduction

MonoMod is a modding "basework" (base tools + framework).
Mods / mod loaders for the following games are already using it in one way or another:

Terraria: tModLoader, TerrariaHooks
Hollow Knight: HollowKnight.Modding
Celeste: Everest
Risk of Rain 2: BepInExPack (BepInEx + MonoMod + R2API)
Enter the Gungeon: Mod the Gungeon
Rain World: RainDB via custom BepInEx package
Totally Accurate Battle Simulator: TABS-Multiplayer
Salt and Sanctuary: Salt.Modding
Nimbatus: Nimbatus-Mods via Partiality
Dungeon of the Endless: DungeonOfTheEndless-Mod via Partiality
FEZ: FEZMod (defunct)
And many more! Ping me on Discord if your mod uses MonoMod!

It consists of the following modular components:

MonoMod: The core MonoMod IL patcher and relinker.
MonoMod.Utils: Utilities and helpers that not only benefit MonoMod, but also mods in general. It contains classes such as FastReflectionHelper, LimitedStream, DynamicMethodHelper, DynamicMethodDefinition, DynDll and the ModInterop namespace.
MonoMod.DebugIL: Enable IL-level debugging of third-party assemblies in Visual Studio / MonoDevelop.
MonoMod.BaseLoader: A base on which a C# mod loader can be built upon, including a basic engine-agnostic mod content manager and mod relinker.
MonoMod.RuntimeDetour: A flexible and easily extensible runtime detouring library, supporting X86+ and ARMv7+, .NET Framework, .NET Core and mono.
HookGen: A utility to generate a "hook helper .dll" for any IL assembly. This allows you to hook methods in runtime mods as if they were events. Built with MonoMod and RuntimeDetour.

Why?

Cross-version compatibility, even with obfuscated assemblies.
Cross-platform compatibility, even if the game uses another engine (f.e. Celeste uses XNA on Windows, FNA on macOS and Linux).
Use language features which otherwise wouldn't be supported (f.e. C# 7 in Unity 4.3).
Patch on the player's machine with a basic mod installer. No need to pre-patch, no redistribution of game data, no copyright violations.
With HookGen, runtime hooks are as simple as On.Namespace.Type.Method += (orig, a, b, c) => { /* ... */ }
With HookGen IL, you can manipulate IL at runtime and even inline C# delegate calls between instructions.
Modularity allows you to mix and match. Use only what you need!

Using MonoMod

Drop MonoMod.exe, all dependencies (Utils, cecil) and your patches into the game directory. Then, in your favorite shell (cmd, bash):

MonoMod.exe Assembly.exe

MonoMod scans the directory for files named [Assembly].*.mm.dll and generates MONOMODDED_[Assembly].exe, which is the patched version of the assembly.

Example Patch

You've got Celeste.exe and want to patch the method public override void Celeste.Player.Added(Scene scene).

If you haven't created a mod project yet, create a shared C# library project called Celeste.ModNameHere.mm, targeting the same framework as Celeste.exe.
Add Celeste.exe, MonoMod.exe and all dependencies (.Utils, cecil) as assembly references.
Note: Make sure to set "Copy Local" to False on the game's assemblies. Otherwise your patch will ship with a copy of the game!

#pragma warning disable CS0626 // orig_ method is marked external and has no attributes on it.
namespace Celeste {
    // The patch_ class is in the same namespace as the original class.
    // This can be bypassed by placing it anywhere else and using [MonoModPatch("global::Celeste.Player")]

    // Visibility defaults to "internal", which hides your patch from runtime mods.
    // If you want to "expose" new members to runtime mods, create extension methods in a public static class PlayerExt
    class patch_Player : Player { // : Player lets us reuse any of its visible members without redefining them.
        // MonoMod creates a copy of the original method, called orig_Added.
        public extern void orig_Added(Scene scene);
        public override void Added(Scene scene) {
            // Do anything before.

            // Feel free to modify the parameters.
            // You can even replace the method's code entirely by ignoring the orig_ method.
            orig_Added(scene);
            
            // Do anything afterwards.
        }
    }
}

Build Celeste.ModNameHere.mm.dll, copy it into the game's directory and run MonoMod.exe Celeste.exe, which generates MONOMODDED_Celeste.exe.
Note: This can be automated by a post-build step in your IDE and integrated in an installer, f.e. Everest.Installer (GUI), MiniInstaller (CLI) or PartialityLauncher (GUI).

To make patching easy, yet flexible, the MonoMod patcher offers a few extra features:

MonoMod.MonoModRules will be executed at patch time. Your rules can define relink maps (relinking methods, fields or complete assemblies), change the patch behavior per platform or define custom modifiers to f.e. modify a method on IL-level using cecil.
For types and methods, to be ignored by MonoMod (because of a superclass that is defined in another assembly and thus shouldn't be patched), use the MonoModIgnore attribute.
Check the full list of standard modifiers with their descriptions, including "patch-time hooks", proxies via [MonoModLinkTo] and [MonoModLinkFrom], conditinal patching via [MonoModIfFlag] + MonoModRules, and a few more.

FAQ

How does the patcher work?

MonoMod first checks for a MonoMod.MonoModRules type in your patch assembly, isolates it and executes the code.
It then copies any new types, including nested types, except for patch types and ignored types.
Afterwards, it copies each type member and patches the methods. Make sure to use [MonoModIgnore] on anything you don't want to change.
Finally, all relink mappings get applied and all references get fixed (method calls, field get / set operations, ...).

How can I check if my assembly has been modded?

MonoMod creates a type called "WasHere" in the namespace "MonoMod" in your assembly:

if (Assembly.GetExecutingAssembly().GetType("MonoMod.WasHere") != null) {
    Console.WriteLine("MonoMod was here!");
} else {
    Console.WriteLine("Everything fine, move on.");
}

Note: This can be easily bypassed. More importantly, it doesn't detect changes made using other tools like dnSpy.
If you're a gamedev worried about cheating: Please don't fight against the modding community. Cheaters will find another way to cheat, and modders love to work together with gamedevs.

Am I allowed to redistribute the patched assembly?

This depends on the licensing situation of the input assemblies. If you're not sure, ask the authors of the patch and of the game / program / library.

Is it possible to use multiple patches?

Yes, as long as the patches don't affect the same regions of code.

While possible, its behaviour is not strictly defined and depends on the patching order.
Instead, please use runtime detours / hooks instead, as those were built with "multiple mod support" in mind.