Unity Doorstop

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Doorstop is a tool to execute managed .NET assemblies inside Unity as early as possible.

Features

• Runs first: Doorstop runs its code before Unity can do so
• Configurable: An elementary configuration file allows you to specify your assembly to execute
• Multiplatform: Supports Windows, Linux, macOS
• Debugger support: Allows to debug managed assemblies in Visual Studio, Rider or dnSpy without modifications to Mono

Unity runtime support

Doorstop supports executing .NET assemblies in both Unity Mono and Il2Cpp runtimes. Depending on the runtime the game uses, Doorstop tries to run your assembly as follows:

• On Unity Mono, your assembly is executed in the same runtime. As a result
  • You don't need to include your custom Common Language Runtime (CLR); the one bundled with the game is used
  • Your assembly is run alongside other Unity code
  • You can access all Unity API directly
• On Il2Cpp, your assembly is executed in CoreCLR runtime because Il2Cpp cannot run managed assemblies. As a result:
  • You need to include .NET 6 or newer CoreCLR runtime with your managed assembly
  • Your assembly is run in a runtime that is isolated from Il2Cpp
  • You can access Il2Cpp runtime by interacting with its native il2cpp_ API

Building

Doorstop uses Zig to build the project. To build, run zig build.

Minimal injection example

To have Doorstop inject your code, create Entrypoint class into Doorstop namespace. Define a public static Start method in it:

using System.IO;

namespace Doorstop;

class Entrypoint
{
  public static void Start()
  {
      File.WriteAllText("doorstop_hello.log", "Hello from Unity!");
  }
}

You can then define any code you want in Start.

NOTE: On UnityMono, Doorstop bootstraps your assembly with a minimal number of assemblies and minimal configuration. This early execution allows for some interesting tricks, like redirecting the loading of some game assemblies. Bear also in mind that some of the Unity runtime is not initialized at such an early stage, limiting the code you can execute. You might need to appropriately pause the execution of your code until the moment you want to modify the game.

Doorstop environment variables

Doorstop sets some environment variables useful for code execution:

Environment variable	Description
DOORSTOP_INITIALIZED	Always set to TRUE. Use to determine if your code is run via Doorstop.
DOORSTOP_INVOKE_DLL_PATH	Path to the assembly executed by Doorstop relative to the current working directory (this part is not even true upstream).
DOORSTOP_PROCESS_PATH	Path to the application executable where the injected assembly is run.
DOORSTOP_MANAGED_FOLDER_DIR	UnityMono: Path to the game's Managed folder. Il2Cpp: Path to CoreCLR's base class library folder.
DOORSTOP_DLL_SEARCH_DIRS	Paths where the runtime searchs assemblies from by default, separated by OS-specific separator (; on windows and : on *nix).
DOORSTOP_MONO_LIB_PATH	Only on UnityMono: Full path to the mono runtime library.

Debugging

Doorstop 4 supports debugging the assemblies in the runtime.

Debugging in UnityMono

To enable debugging, set debug_enabled to true and optionally change the debug server address via debug_address (see configuration options). After launching the game, you may connect to the debugger using the server address (default is 127.0.0.1:10000). By default, the game won't wait for the debugger to connect; you may change the behaviour with the debug_suspend option.

If you use dnSpy, you can use the Debug > Start Debugging > Debug engine > Unity option, automatically setting the correct debugging configuration. Doorstop detects dnSpy and automatically enables debugging without any extra configuration.

Debugging in Il2Cpp

Debugging is automatically enabled in CoreCLR.

To start debugging, compile your DLL in debug mode (with embedded or portable symbols) and start the game with the debugger of your choice. Alternatively, attach a debugger to the game once it is running. All standard CoreCLR debuggers should detect the CoreCLR runtime in the game.

Moreover, hot reloading is supported for Visual Studio, Rider and other debuggers with .NET 6 hot reloading feature enabled.

Note that you can only debug managed code! Because the game code is unmanaged (i.e. Il2Cpp), you cannot directly debug the actual game code. Consider using native debuggers like GDB and visual debugging tools like IDA or Ghidra to debug actual game code.

Doorstop configuration

Doorstop is reasonably configurable via environment variables.

The following environment variables are consumed by Doorstop:

Platform specific variables will be indicated as such.

• bool = 1 or 0
• string = any valid environment variable value.
• path = an absolute path

Name	Type	Description
DOORSTOP_ENABLED	bool	Enable or disable Doorstop.
DOORSTOP_REDIRECT_OUTPUT_LOG	bool	Only on Windows: If true Unity's output log is redirected to <current folder>\output_log.txt
DOORSTOP_TARGET_ASSEMBLY	path	Path to the assembly to load and execute.
DOORSTOP_BOOT_CONFIG_OVERRIDE	path	Overrides the boot.config file path.
DOORSTOP_MONO_DLL_SEARCH_PATH_OVERRIDE	string	Overrides default Mono DLL search path
DOORSTOP_MONO_DEBUG_ENABLED	bool	If true, Mono debugger server will be enabled
DOORSTOP_MONO_DEBUG_SUSPEND	bool	Whether to suspend the game execution until the debugger is attached.
DOORSTOP_MONO_DEBUG_ADDRESS	string	The address to use for the Mono debugger server.
DOORSTOP_CLR_CORLIB_DIR	path	Path to coreclr library that contains the CoreCLR runtime
DOORSTOP_CLR_RUNTIME_CORECLR_PATH	path	Path to the directory containing the managed core libraries for CoreCLR (mscorlib, System, etc.)

License

Doorstop 4 is licensed under LGPLv2.1. You can view the entire license here.