Runtime exception with ConditionalWeakTable, Unity Mono IL2CPP

[CreepGin]

After PR #1227, we are getting the following runtime exception on AOT platforms like Unity IL2CPP.

InvalidCastException: Unable to cast object of type 'FileInfo' to type 'ObjectWrapper'.
  at System.Runtime.CompilerServices.ConditionalWeakTable`2[TKey,TValue].TryGetValue (TKey key, TValue& value) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.InteropOptions+<>c.<.ctor>b__59_0 (Jint.Engine engine, System.Object target) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.DefaultObjectConverter.TryConvert (Jint.Engine engine, System.Object value, Jint.Native.JsValue& result) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.Native.JsValue.FromObject (Jint.Engine engine, System.Object value) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.Native.Object.ObjectInstance.Get (Jint.Native.JsValue property, Jint.Native.JsValue receiver) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.Runtime.Interop.ObjectWrapper.Get (Jint.Native.JsValue property, Jint.Native.JsValue receiver) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.Engine.GetValue (Jint.Runtime.References.Reference reference, System.Boolean returnReferenceToPool) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.Runtime.Interpreter.Expressions.JintExpression.GetValue (Jint.Runtime.Interpreter.EvaluationContext context) [0x00000] in <00000000000000000000000000000000>:0 
  at Jint.Runtime.Interpreter.Expressions.JintLogicalOrExpression.EvaluateInternal (Jint.Runtime.Interpreter.EvaluationContext context) [0x00000] in <00000000000000000000000000000000>:0 
...

I think the offending line is

jint/Jint/Options.cs

Line 258 in 3eddf04

if (engine._objectWrapperCache.TryGetValue(target, out var wrapped))

After a bit of digging, I'm beginning to suspect that Unity/Mono doesn't work well with ConditionalWeakTable.

[lahma]

I'm at lost with these as I don't do Unity development, is it enough for you to disable the functionality with the switch engine.Options.Interop.TrackObjectWrapperIdentity = false ?

[CreepGin]

Yes, that is what we did for the time-being. And I understand there's nothing much you can do in this case.

The exception with ConditionalWeakTable doesn't happen in Unity Editor. Only happens in deployed builds (and doesn't reproduce consistently since Unity/Mono's ConditionalWeakTable gets "corrupted" differently on difference machines).

So I think Unity devs using Jint will get tripped by this eventually. But at least now they can search Google and arrive at this thread. 😀

_engine = new Jint.Engine(opts => {
    // Jint uses ConditionalWeakTable for tracking ObjectWrapper identity, but Unity is buggy with ConditionalWeakTable.
    opts.Interop.TrackObjectWrapperIdentity = false;
    ...
}

[lahma]

When hopefully eventually have some documentation, we can have some sort unity specific section for best practices..