System.InvalidCastException: 'Unable to cast object of type 'System.Object[]' to type 'System.String'.'

[klangille]

Hi,

I'm in process of updated my project to version 8.14.0 and getting this exception:

System.InvalidCastException: 'Unable to cast object of type 'System.Object[]' to type 'System.String'.'
This exception was originally thrown at this call stack:
    IKVM.Runtime.StubGen.StubGenerator.EncodeElementValue.AnonymousMethod__3(IKVM.ByteCode.Encoding.ElementValuePairTableEncoder)
    IKVM.ByteCode.Encoding.AnnotationEncoder.Annotation(IKVM.ByteCode.Utf8ConstantHandle, System.Action<IKVM.ByteCode.Encoding.ElementValuePairTableEncoder>)
    IKVM.Runtime.StubGen.StubGenerator.EncodeElementValue.AnonymousMethod__1(IKVM.ByteCode.Encoding.AnnotationEncoder)
    IKVM.ByteCode.Encoding.ElementValueEncoder.Annotation(System.Action<IKVM.ByteCode.Encoding.AnnotationEncoder>)
    IKVM.Runtime.StubGen.StubGenerator.EncodeElementValue(IKVM.ByteCode.Encoding.ClassFileBuilder, ref IKVM.ByteCode.Encoding.ElementValueEncoder, object)
    IKVM.Runtime.StubGen.StubGenerator.EncodeElementValue.AnonymousMethod__2(IKVM.ByteCode.Encoding.ElementValueEncoder)
    IKVM.ByteCode.Encoding.ElementValueTableEncoder.ElementValue(System.Action<IKVM.ByteCode.Encoding.ElementValueEncoder>)
    IKVM.Runtime.StubGen.StubGenerator.EncodeElementValue.AnonymousMethod__0(IKVM.ByteCode.Encoding.ElementValueTableEncoder)
    IKVM.ByteCode.Encoding.ElementValueEncoder.Array(System.Action<IKVM.ByteCode.Encoding.ElementValueTableEncoder>)
    IKVM.Runtime.StubGen.StubGenerator.EncodeElementValue(IKVM.ByteCode.Encoding.ClassFileBuilder, ref IKVM.ByteCode.Encoding.ElementValueEncoder, object)
    ...
    [Call Stack Truncated]

I'm not super familiar with the IKVM source but ChatGPT assures me the fix is the have the line in StubGenerator.EncodeElementValue changed because the array is name value pairs and should be advancing by 2.

from: for (int i = 1; i < v.Length; i++)
to: for (int i = 2; i < v.Length; i += 2)

[wasabii]

Nice catch — you nailed it. That's exactly the bug, and your diagnosis of the pairwise iteration is right.

The one detail is that the loop already starts at i = 2; the only thing wrong is the increment. It's walking name/value pairs but stepping by one (i++) instead of two (i += 2), so after the first pair i lands on a value slot and the (string)v[i] cast blows up the moment that value is an array — a nested annotation, an enum, or an array member. Which is why it only shows up for nested annotations with more than one element; the single-element case happens to work by accident.

As for how it got there: it's a regression from 3dad0f9 (Aug 18, 2024), when the stub generator was rewritten on top of IKVM.ByteCode. The old code encoded these with i += 2 correctly. The rewrite ended up with three copies of the same pairwise loop, and two of them (EncodeAnnotation and the CustomAttributeTypedArgument overload) kept the += 2 — this one didn't. So it's been sitting in there for about two years, just waiting for someone to feed it a multi-element nested annotation.

Fix is up in #725. Thanks for the clear report.

[klangille]

Awesome, thanks for the quick reply. A couple of questions

1. What is the easiest way to use this fix so I can continue on with my upgrade until it is release?
2. When is the next stable release planned?