CVector | A ShiViz-compatible logging library for C

This would be the correct syntax. This needs to be placed outside of main():

As the comments mention, there is no way to use an index setter, and as far as I can see, the indexer always throws IndexOutOfRangeException for valuetuple on .net 4.8

The obvious solution would be to write your own indexer:

Copied and extended from my earlier comment.

You may have a faulty cast or poke. One thing I make a point of doing, both as a defensive guideline and when debugging, is this:

Explicitly annotate the type of everything that can undermine types. That way, you always know what you’re getting. Even if a poke, castPtr, or unsafeCoerce has my intended type now, that may not be stable under code motion. And even if this doesn’t identify the issue, it can at least help think through it.

For example, I was once writing a null terminator into a byte buffer…which corrupted adjacent memory by writing beyond the end, because I was using '\NUL', which is not a char, but a Char—32 bits! The reason was that pokeByteOff is polymorphic: it has type (Storable a) => Ptr b -> Int -> a -> IO (), not … => Ptr a -> ….

This turned out to be the case in your code! Quoth @aclow:

The createVector generated by c2hs was equivalent to something like alloca $ \ ptr -> createCVector'_ ptr >> peek ptr, where createCVector'_ :: Ptr () -> IO (), which meant that alloca allocated only enough space to hold a unit. Changing the in-marshaller to alloca' f = alloca $ f . (castPtr :: Ptr ForeignVector -> Ptr ()) seems to solve the issue.

Things that turned out not to be the case, but could’ve been:

I’ve encountered a similar crash when a closure was getting corrupted by somebody (read: me) writing beyond an array. If you’re doing any writes without bounds checking, it may be helpful to replace them with checked versions to see if you can get an exception rather than heap corruption. In a way this is what was happening here, except that the write was to the alloca-allocated region, not the array.

Alternatively, consider lifetime issues: whether the ForeignPtr could be getting dropped & freeing the buffer earlier than you expect, giving you a use-after-free. In a particularly frustrating case, I’ve had to use touchForeignPtr to keep a ForeignPtr alive for that reason.

You're trying to free memory you didn't malloc. And you don't free the one piece of memory you did malloc. In your case you probably meant: