Archival note

This repository has been unmaintained for a while and will be archived soon. Consider using BepInEx/Il2CppInterop as a replacement.

Il2CppAssemblyUnhollower

A tool to generate Managed->IL2CPP proxy assemblies from Il2CppDumper's output.

This allows the use of IL2CPP domain and objects in it from a managed domain. This includes generic types and methods, arrays, and new object creation. Some things may be horribly broken.

Usage

0. Build or get a release
1. Obtain dummy assemblies using Il2CppDumper
2. Run AssemblyUnhollower --input=<path to Il2CppDumper's dummy dll dir> --output=<output directory> --mscorlib=<path to target mscorlib>

Resulting assemblies may be used with your favorite loader that offers a Mono domain in the IL2CPP game process, such as MelonLoader.
This appears to be working reasonably well for Unity 2018.4.x games, but more extensive testing is required.
Generated assemblies appear to be invalid according to .NET Core/.NET Framework, but run fine on Mono.

Command-line parameter reference

Usage: AssemblyUnhollower [parameters]
Possible parameters:
        --help, -h, /? - Optional. Show this help
        --verbose - Optional. Produce more console output
        --input=<directory path> - Required. Directory with Il2CppDumper's dummy assemblies
        --output=<directory path> - Required. Directory to put results into
        --mscorlib=<file path> - Required. mscorlib.dll of target runtime system (typically loader's)
        --unity=<directory path> - Optional. Directory with original Unity assemblies for unstripping
        --gameassembly=<file path> - Optional. Path to GameAssembly.dll. Used for certain analyses
        --deobf-uniq-chars=<number> - Optional. How many characters per unique token to use during deobfuscation
        --deobf-uniq-max=<number> - Optional. How many maximum unique tokens per type are allowed during deobfuscation
        --deobf-analyze - Optional. Analyze deobfuscation performance with different parameter values. Will not generate assemblies.
        --blacklist-assembly=<assembly name> - Optional. Don't write specified assembly to output. Can be used multiple times
        --add-prefix-to=<assembly name/namespace> - Optional. Assemblies and namespaces starting with these will get an Il2Cpp prefix in generated assemblies. Can be used multiple times.
        --no-xref-cache - Optional. Don't generate xref scanning cache. All scanning will be done at runtime.
        --no-copy-unhollower-libs - Optional. Don't copy unhollower libraries to output directory
        --obf-regex=<regex> - Optional. Specifies a regex for obfuscated names. All types and members matching will be renamed
        --rename-map=<file path> - Optional. Specifies a file specifying rename map for obfuscated types and members
        --passthrough-names - Optional. If specified, names will be copied from input assemblies as-is without renaming or deobfuscation
Deobfuscation map generation mode:
        --deobf-generate - Generate a deobfuscation map for input files. Will not generate assemblies.
        --deobf-generate-asm=<assembly name> - Optional. Include this assembly for deobfuscation map generation. If none are specified, all assemblies will be included.
        --deobf-generate-new=<directory path> - Required. Specifies the directory with new (obfuscated) assemblies. The --input parameter specifies old (unobfuscated) assemblies. 

Required external setup

Before certain features can be used (namely class injection and delegate conversion), some external setup is required.

• Set ClassInjector.Detour to an implementation of a managed detour with semantics as described in the interface
• Alternatively, set ClassInjector.DoHook to an Action with same semantics as DetourAttach (signature void**, void*, first is a pointer to a variable containing pointer to hooked code start, second is a pointer to patch code start, a pointer to call-original code start is written to the first parameter)
• Call UnityVersionHandler.Initialize with appropriate Unity version (default is 2018.4.20)

Known Issues

• Non-blittable structs can't be used in delegates
• Types implementing interfaces, particularly IEnumerable, may be arbitrarily janky with interface methods. Additionally, using them in foreach may result in implicit casts on managed side (instead of Cast<T>, see below), leading to exceptions. Use var in foreach or use for instead of foreach when possible as a workaround, or cast them to the specific interface you want to use.
• in/out/ref parameters on generic parameter types (like out T in Dictionary.TryGetValue) are currently broken
• Unity unstripping only partially restores types, and certain methods can't be unstripped still; some calls to unstripped methods might result in crashes
• Unstripped methods with array operations inside contain invalid bytecode
• Unstripped methods with casts inside will likely throw invalid cast exceptions or produce nulls
• Some unstripped methods are stubbed with NotSupportedException in cases where rewrite failed
• Nullables have issues when returned from field/property getters and methods

Generated assemblies caveats

• IL2CPP types must be cast using .Cast<T> or .TryCast<T> methods instead of C-style casts or as.
• When IL2CPP code requires a System.Type, use Il2CppType.Of<T>() instead of typeof(T)
• For IL2CPP delegate types, use the implicit conversion from System.Action or System.Func, like this: UnityAction a = new Action(() => {}) or var x = (UnityAction) new Action(() => {})
• IL2CPP assemblies are stripped, so some methods or even classes could be missing compared to pre-IL2CPP assemblies. This is mostly applicable to Unity assemblies.
• Using generics with value types may lead to exceptions or crashes because of missing method bodies. If a specific value-typed generic signature was not used in original game code, it can't be used externally either.

Class injection

Starting with version 0.4.0.0, managed classes can be injected into IL2CPP domain. Currently this is fairly limited, but functional enough for GC integration and implementing custom MonoBehaviors.

How-to:

• Your class must inherit from a non-abstract IL2CPP class.
• You must include a constructor that takes IntPtr and passes it to base class constructor. It will be called when objects of your class are created from IL2CPP side.
• To create your object from managed side, call base class IntPtr constructor with result of ClassInjector.DerivedConstructorPointer<T>(), where T is your class type, and call ClassInjector.DerivedConstructorBody(this) in constructor body.
• An example of injected class is Il2CppToMonoDelegateReference in DelegateSupport.cs
• Call ClassInjector.RegisterTypeInIl2Cpp<T>() before first use of class to be injected
• The injected class can be used normally afterwards, for example a custom MonoBehavior implementation would work with AddComponent<T>

Fine-tuning:

• [HideFromIl2Cpp] can be used to prevent a method from being exposed to il2cpp

Caveats:

• Injected class instances are handled by IL2CPP garbage collection. This means that an object may be collected even if it's referenced from managed domain. Attempting to use that object afterwards will result in ObjectCollectedException. Conversely, managed representation of injected object will not be garbage collected as long as it's referenced from IL2CPP domain.
• It might be possible to create a cross-domain reference loop that will prevent objects from being garbage collected. Avoid doing anything that will result in injected class instances (indirectly) storing references to itself. The simplest example of how to leak memory is this:

class Injected: Il2CppSystem.Object {
    Il2CppSystem.Collections.Generic.List<Il2CppSystem.Object> list = new ...;
    public Injected() {
        list.Add(this); // reference to itself through an IL2CPP list. This will prevent both this and list from being garbage collected, ever.
    }
}

Limitations:

• Interfaces can't be implemented
• Virtual methods can't be overridden
• Only instance methods are exposed to IL2CPP side - no fields, properties, events or static methods will be visible to IL2CPP reflection
• Only a limited set of types is supported for method signatures

Injected components in asset bundles

Starting with version 0.4.15.0, injected components can be used in asset bundles. Currently, deserialization for component fields is not supported. Any fields on the component will initially have their default value as defined in the mono assembly.

How-to:

• Your class must meet the above critereon mentioned in Class Injection.
• Add a dummy script for your component into Unity. Remove any methods, constructors, and properties. Fields can optionally be left in for future deserialization support.
• Apply the component to your intended objects in Unity and build the asset bundle.
• At runtime, register your component with RegisterTypeInIl2Cpp before loading any objects from the asset bundle.

Implementing interfaces with injected types

Starting with 0.4.16.0, injected types can implement IL2CPP interfaces.
Just like previously, your type can't implement the interface directly, as it's still generated as a class.
However, you can pass additional interface types to RegisterTypeInIl2CppWithInterfaces, and they will be implemented as interfaces on the IL2CPP version of your type.
Interface methods are matched to methods in your class by name, parameter count and genericness.
Known caveats:

• obj.Cast<InterfaceType>() will fail if you try to cast an object of your injected type to an interface. You can work around that with new InterfaceType(obj.Pointer) if you're absolutely sure it implements that interface.
• Limited method matching might result in some interfaces being trickier or impossible to implement, namely those with overloads differing only in parameter types.

PDB generator

UnhollowerPdbGen builds an executable that can be ran to generate a Microsoft PDB file (debug symbols) for GameAssembly.dll based on unhollower-generated names.
This can be useful for analyzing code of obfuscated games. For unobfuscated games, using Il2CppInspector would provide way better results for code analysis.
Generated PDBs were tested with windbg, lldb, WPA viewer/ETL performance analysis and IDA.
Generated PDBs only include generated methods, and don't include type info, generic method info and IL2CPP internals.
You need to manually copy the following Microsoft-provided libraries from Visual Studio (or other build tools) for this to work - I'm not redistributing them as license on them is not clear.

• mspdbcore.dll
• msobj140.dll
• tbbmalloc.dll

These need to be placed next to the built .exe file. Use file search to find mspdbcore in VS install.

Upcoming features (aka TODO list)

• Unstripping engine code - fix current issues with unstripping failing or generating invalid bytecode
• Proper interface support - IL2CPP interfaces will be generated as interfaces and properly implemented by IL2CPP types
• Improve class injection to support virtual methods and interfaces
• Improve class injection to support deserializing fields

Used libraries

Bundled into output files:

• iced by 0xd4d, an x86 disassembler used for xref scanning and possibly more in the future

Used by generator itself:

• Mono.Cecil by jbevain, the main tool to produce assemblies

Parts of source used:

• microsoft-pdb for the PDB generator