Using preloader patchers


As of version 4.0, BepInEx allows to write preload-time patchers that modify assemblies before the game loads them.
While most plug-ins can use Harmony to do runtime patching, using preload-time patchers provides more fine control over how the assembly is patched.

It is still recommended that you use Harmony wherever possible because Harmony makes sure all patches are compatible with each other. Use Mono.Cecil only if something cannot be done by Harmony (more info below).

Note: The contract for preloader patchers has changed between BepInEx v5 and v6.

Difference from runtime patchers

Because preload-time patchers are run before the assemblies are loaded into memory, the patchers have more fine-grained control over how to modify the assemblies.

Feature Preload-time patcher Runtime patcher
Used library Mono.Cecil Harmony
Used contract Written in a separate DLL, uses a special contract Written in plug-in DLL, uses Harmony's API
Application time Applied on raw assemblies before the game initializes Applied on assemblies already loaded in memory
Can apply hooks Yes Yes, as long as the target is not inlined by JIT
Can reference game assembly directly No Yes
Can rewrite methods' IL Yes Yes
Can modify field/method propeties Everything Partially
Can add new classes, methods and fields Yes No
Can replace assemblies Yes No

Thus, use preload-time patchers only if you must modify the structure of the assembly. For hooking methods use Harmony.


Preloader-time patching comes with its own caveats! Refer to the notes below for more information.

Writing a patcher


Assuming you know how to use an IDE of your choice, you will need to

Patcher plugin

A patcher plugin's skeleton is similar to a regular plugin:

[PatcherPluginInfo("io.bepis.mytestplugin", "My Test Plugin", "1.0")]
class EntrypointPatcher : BasePatcher
    public override void Initialize() { }

    public override void Finalizer() { }


Notable things:

You have access to the same base properties that regular plugins do; i.e. Log, Config and and Info. You also have access to Context, which is an object that contains the current information that the assembly patcher engine within BepInEx is currently using. For example, you can use it to find out which other patcher plugins are loaded, which assemblies can be patched, which patches have already been applied etc.

Note that your patcher plugin GUID must be unique, even against regular plugins! Because patcher plugins have their own configuration files now, they must also have a unique GUID so that there aren't any conflicts when loading / saving configuration settings.


This is the lifecycle of the patcher engine within BepInEx:

  1. All .dll files within BepInEx/patchers are examined to see if they contain any patcher plugins. The ones that do are loaded as assemblies.
  2. Every discovered patcher plugin is instantiated once (by calling the constructor).
  3. All patcher plugins have their Initialize() function called.
  4. Every patching method within each patcher plugin is executed, against the targeted type / assembly. Any unhandled exceptions are logged.
  5. All patcher plugins have their Finalizer() function called.
  6. Patcher engine unloads all loaded AssemblyDefinition and TypeDefinition objects.

Use your Initialize method for code that needs to run first exactly once, and your Finalizer method for code that needs to run last exactly once.

Patch methods

Patch methods are much more declarative now, very similar to declaring Harmony patches. Here is an example declaration:

public void PatchAssembly(AssemblyDefinition assembly)

You can target assemblies, or specific types (detailed below).

Patch methods must not be static or abstract. They can be any visibility, however.

They can have void or bool as a return type. In the case of bool, the return value specifies if the targeted assembly or type has been modified by the patcher. This is important, because if you tell BepInEx that the patch method hasn't actually patched anything, then it won't mark the assembly / types you've requested as modified. With a void return type, BepInEx will always assume that you have performed modifications.

If you have an AssemblyDefinition as the first parameter, then you can also define it as ref if you wish to replace it with another definition entirely. This is useful if you want to replace an assembly with another one you have shipped yourself, for example.

You can also provide a second string parameter, which will contain the (relative) filename of the assembly. If you are targeting a type, then it will return the filename of the assembly that the type belongs to.

For patch methods that target assemblies, you can specify multiple assemblies:

public void PatchAssembly(AssemblyDefinition assembly, string filename)

Which will then run that patch method twice, once for each assembly. There is also the option of specifying all available assemblies:

public void PatchAssembly(AssemblyDefinition assembly, string filename)

As stated above you also have the option of specifying specific types. For example:

[TargetType("Assembly-CSharp.dll", "GameNamespace.GameClass")]
public void PatchAssembly(TypeDefinition type)

The first parameter of the attribute is the filename of the assembly where the type belongs, and the second parameter is the full name of the type you wish to patch (including namespaces).

Notes and tips