Memory Access Violation on `Il2CppSystem.Enum.ToString`

[AndrewSav]

I was working on a mod for Blue Prince and I noticed that the process was crashing with memory access violation when calling ToString on instances of Il2CppSystem.Enum returned from the game. Not on a single one of them, on all that I tried.

I tried BepInEx and it did not crash under the same conditions. A MelonLoader discord member mentioned that the generation is the same in both, so that should not be happening. I compared the version Il2CppInterop used in both projects, and while they both are 1.4.6 they are different. MelonLoader uses 1.4.6-ci.579 whereas Bepinex uses 1.4.6-ci.426. Interestingly enough the 426 nuget package does not have the corresponding commit information, and the GitHub actions build makes it look like it came from a fork/branch since deleted.

None the less I was able to find a commit in the history that still worked, and bisected, until I found that the big 06cd048 commit is where it changed and stopped working.

When debugging, this is what I'm seeing when it crashes (on builds after the commit):

And this is what it looks like when it does not crash (before the commit):

I also noticed that type of Il2CppSystem.Enum changed from class to struct. Any idea what I'm doing wrong, and how to fix this?

[ds5678]

I also noticed that type of Il2CppSystem.Enum changed from class to struct

This is the problem.

If you make a pull request, I'll review and potentially merge it. In regards to MelonLoader, the developers are generally responsive with Il2CppInterop version bumps, so your change could be accessible in nightly builds with little delay. This is probably where you need to fix the problem:

https://github.com/BepInEx/Il2CppInterop/blob/763f4279f456feac12c44cd2679e56f12d193610/Il2CppInterop.Generator/Passes/Pass12FillTypedefs.cs

I looked over the AsmResolver commit and did not see a reason why behavior should have changed. In fact, I did extensive IL validation during development and directly before the pull request got merged. It caught the most subtle of differences, so I expect that the IL difference between those two screenshots would have certainly been caught. It makes me doubt your screenshots a little.

For full disclosure, I'm currently rewriting the generator, and your issue is not present in the current output.

[AndrewSav]

@ds5678 is it clear, that this is observed on Il2CppSystem.Enum / System.Enum when processing mscorlib.dll ?

[ds5678]

I verified that the issue presents using the current codebase. I just doubt that particular commit caused the issue.

[AndrewSav]

@ds5678 so Cecil thinks that System.Enum is reference type and AsmResover thinks it's a value type, and then:

Il2CppInterop/Il2CppInterop.Generator/Contexts/TypeRewriteContext.cs

Lines 51 to 52 in 06cd048

	if (!OriginalType.IsValueType)
	ComputedTypeSpecifics = TypeSpecifics.ReferenceType;

Here is your difference. What do you want to do about it?

I can add a check here for mscorlib and System.Enum and return TypeSpecifics.ReferenceType in this case, but I'm not sure this is a correct solution.

[AndrewSav]

I opened an issue for clarification here: Washi1337/AsmResolver#635

[ds5678]

Fixing it in AsmResolver is indeed the correct solution.

[AndrewSav]

This got merged, but it could take a while until the next release

[ds5678]

Since a proper fix has been implemented upstream, I'm okay with a temp fix here in the meantime.

[AndrewSav]

It is still crashing on this line with memory access violation, despite this is now a class not a struct. Over to you @ds5678

Update: I think my patch is not enough - the generator must be checking the IsValueType also somewhere too, so some other thing is generated wrong too. I'll see if I can figure this out.

[ds5678]

Your intuition is correct. The problem is the unbox call.

[AndrewSav]

Alright. Hold on to your socks #212

[ds5678]

Thanks for reporting this and implementing a fix.