Transpiles fail in Unity 2019.4.27, HarmonyX 2.5.5

The following error occurs when trying to use transpile patches on any method. Prefix and Postfix patches work fine.

Game : Outer Wilds (Steam) v1.1.10 OS : Windows 10 HarmonyX Version : 2.5.5

Exception while patching DebugInputManager.Awake: HarmonyLib.HarmonyException: IL Compile Error (unknown location) ---> HarmonyLib.HarmonyException: IL Compile Error (unknown location) ---> System.BadImageFormatException: Method has zero rva
  at (wrapper managed-to-native) System.RuntimeMethodHandle.GetFunctionPointer(intptr)
  at System.RuntimeMethodHandle.GetFunctionPointer () [0x00000] in <44afb4564e9347cf99a1865351ea8f4a>:0
  at MonoMod.RuntimeDetour.Platforms.DetourRuntimeILPlatform.GetFunctionPointer (System.Reflection.MethodBase method, System.RuntimeMethodHandle handle) [0x00000] in <953a8b1875364c7e892cef5080bf5687>:0
  at MonoMod.RuntimeDetour.Platforms.DetourRuntimeILPlatform.GetNativeStart (System.Reflection.MethodBase method) [0x00019] in <953a8b1875364c7e892cef5080bf5687>:0
  at MonoMod.RuntimeDetour.DetourHelper.GetNativeStart (System.Reflection.MethodBase method) [0x00005] in <953a8b1875364c7e892cef5080bf5687>:0
  at MonoMod.RuntimeDetour.Detour._TopApply () [0x00025] in <953a8b1875364c7e892cef5080bf5687>:0
  at MonoMod.RuntimeDetour.Detour._RefreshChain (System.Reflection.MethodBase method) [0x00151] in <953a8b1875364c7e892cef5080bf5687>:0
  at MonoMod.RuntimeDetour.Detour.Apply () [0x00053] in <953a8b1875364c7e892cef5080bf5687>:0
  at MonoMod.RuntimeDetour.Detour..ctor (System.Reflection.MethodBase from, System.Reflection.MethodBase to, MonoMod.RuntimeDetour.DetourConfig& config) [0x002ec] in <953a8b1875364c7e892cef5080bf5687>:0
  at DMD<Refresh>?-2143303168._MonoMod_RuntimeDetour_ILHook+Context::Refresh (MonoMod.RuntimeDetour.ILHook+Context this) [0x0012d] in <a896941d614b4ac9a9d401eaea161531>:0
  at DMD<>?-2143303168.Trampoline<MonoMod.RuntimeDetour.ILHook+Context::Refresh>?-1076096000 (System.Object ) [0x00020] in <19b6e05bfec045fea901aa4a660b03c1>:0
  at HarmonyLib.Internal.RuntimeFixes.StackTraceFixes.OnILChainRefresh (System.Object self) [0x00000] in <7f26c0a74c5b43c2a5fc5efd29ec63d6>:0
  at MonoMod.RuntimeDetour.ILHook.Apply () [0x00059] in <953a8b1875364c7e892cef5080bf5687>:0
  at HarmonyLib.Public.Patching.ManagedMethodPatcher.DetourTo (System.Reflection.MethodBase replacement) [0x00047] in <7f26c0a74c5b43c2a5fc5efd29ec63d6>:0
   --- End of inner exception stack trace ---
  at HarmonyLib.Public.Patching.ManagedMethodPatcher.DetourTo (System.Reflection.MethodBase replacement) [0x0005f] in <7f26c0a74c5b43c2a5fc5efd29ec63d6>:0
  at HarmonyLib.PatchFunctions.UpdateWrapper (System.Reflection.MethodBase original, HarmonyLib.PatchInfo patchInfo) [0x00033] in <7f26c0a74c5b43c2a5fc5efd29ec63d6>:0
   --- End of inner exception stack trace ---
  at HarmonyLib.PatchFunctions.UpdateWrapper (System.Reflection.MethodBase original, HarmonyLib.PatchInfo patchInfo) [0x0005d] in <7f26c0a74c5b43c2a5fc5efd29ec63d6>:0
  at HarmonyLib.PatchProcessor.Patch () [0x000fb] in <7f26c0a74c5b43c2a5fc5efd29ec63d6>:0
  at HarmonyLib.Harmony.Patch (System.Reflection.MethodBase original, HarmonyLib.HarmonyMethod prefix, HarmonyLib.HarmonyMethod postfix, HarmonyLib.HarmonyMethod transpiler, HarmonyLib.HarmonyMethod finalizer, HarmonyLib.HarmonyMethod ilmanipulator) [0x00031] in <7f26c0a74c5b43c2a5fc5efd29ec63d6>:0

The code we use is as follows : Transpile(methodInfo, typeof(Patches), nameof(Patches.EmptyMethod));

public static IEnumerable<CodeInstruction> EmptyMethod(IEnumerable<CodeInstruction> _) => 
  		new List<CodeInstruction>();

The IL code of the method being patched is this :

[misternebula]

Yeah, inserting a single ret fixed this issue. Thanks for the surprisingly easy fix! 😛

[ghorsington]

This could indeed be true. Harmony 2 in itself introduced some changes to how patches are generated. I’ll have a proper look, but I’m fairly sure Harmony 1 always inserted an extra ret to the end of the method, which just happened to not cause issues when you empty a method.

This should be fixable on our side. As such, I’ll keep this issue open and close it once we have a fix for this by automatically adding a ret if a transpiler happened to (brutally) empty a method.

As a side note (for future cases), it’s much more modder-friendly to instead use a prefix with “run original” flag set to false. In other words, it’s enough to have

static bool Prefix()
{
    return false;
}

This will skip the original method code but will still allow other transpilers/code to properly inspect the original IL.