Custom generic collection marshalling with LibraryImport

[kurabirko]

I am working on a project where we have a few C APIs that are shaped like these:

struct DataType1Result {
  DataType1* data;
  size_t size;
};

struct DataType2Result {
  DataType2* data;
  size_t size;
};

extern "C" {
  __declspec(dllexport) DataType1Result* __cdecl fetch_data_1();
  __declspec(dllexport) DataType2Result* __cdecl fetch_data_2();
}

I want to import them over using PInvoke so I wrote custom marshallers for DataType1 and DataType2 etc. Now I want to create marshaller that take a pointer to something like the ListResult<TResult> below and give me an array TResult[] or a list List<TResult>.

readonly struct ListResult<TResult> {
    readonly TResult* Data;
    readonly uint Count;
}

I tried using the stateless unamanged to managed shape and its guaranteed variation but could not get it to work. I'm honestly having a hard time understanding the big picture especially how the marshaller shapes work for collections (i.e. which CustomMarshaller combinations should I be using, how I should be implementing GetManagedValuesDestination or GetUnmanagedValuesSource for my ListResult) even after reading the documentation and the corresponding tutorial. I'd really appreciate it if you could point me towards a reference implementation or a guide etc.