MonoMod

Yet another C# / Mono / .NET assembly patcher using Mono.Cecil.

Special thanks to my patrons on Patreon:

What does it do?

It patches any C# assembly (PE32 executable (console) Intel 80386 Mono/.Net assembly) with any given assembly patches.

The patches are in form of C# assemblies, making it easy to use for quick and (as long as the input assembly isn't obfuscated) version-independent patching / modding / prototyping.

Finally, MonoMod can do any special magic using either the default set of "modifiers" or your own modifiers using "MonoModRules" ("rules" executed by MonoMod at patch time, see below).

How to use it?

You drop the MonoMod.exe, Mono.Cecil.dll and your patches into a folder. Then, in your favorite shell (cmd, bash):

MonoMod.exe Assembly.exe

MonoMod then scans the directory the file is in for files named [Assembly].*.mm.dll (in case of FEZ.exe: FEZ.Mod.mm.dll). Finally, it produces MONOMODDED_[Assembly].exe, which is the patched version of the assembly.

How does it exactly work?

(Optional:) MonoMod checks for the MonoMod.MonoModRules type in your patch assembly, isolates it and executes the code.
It copies any new types to the target assembly that don't exist.
Then it patches each type member: Fields, properties, methods. If they don't exist and aren't ignored, they get added.
Finally, the MonoMod "relinker" applies any relink maps (if specified) and fixes any references (method calls, field get / set operations).

For modders: How to create a patch assembly?

You decompile the input assembly to get a tree of all types and methods. Use that tree to create another assembly which contains pieces of that tree with custom code.

To further simplify patching, MonoMod offers following features:

MonoMod.MonoModRules will be executed at patch time. They can be used to define relink maps (relinking methods, fields or complete assemblies), change the patch behavior per platform or to define custom modifiers.
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.
For types that must have another name than the original type but should still get patched into the input assembly by MonoMod, prefix the type with patch_ or use the MonoModPatch attribute.
To call the original method, create a stub method of the same signature with the prefix orig_ or use the MonoModOriginalName + MonoModOriginal attributes.
Check the full list of modifiers including their descriptions.

For gamedevs and others: How can I check my assembly has been modded?

MonoMod creates a type called "WasHere" in the namespace "MonoMod" in your assembly. You can simply call

    if (Assembly.GetExecutingAssembly().GetType("MonoMod.WasHere") != null) {
        // We Interrupt This Programme
    }

to check if your game has been modded.

Are there limits?

Yes.

Any patch that doesn't compile due to any reasons (feel free to add your workaround here).
Any patch that crashes MonoMod (feel free to add your fix here).
Any patch that causes issues at IL level (feel free to add your black magic here).

Am I allowed to redistibute the patched assembly?

Ask the creator of the input assembly, the patches and your lawyer (actually not your lawyer).

Is it possible to patch patches?

Yes, but the orig_ methods will be ignored by MonoMod.

Is it possible to patch patched patches?

...

Is it possible to make my PC burn?

Maybe. I've heard AMD CPUs start smoking at 400°C.

nochristrequired / MonoMod