PythonNet: How to convert a Python defined class into a c# class when calling C# from Python?

Question

I have a C# app which calls a Python method (Python 3.9.6), which internally calls back into my C# application. The C# dll is being imported into python using from/import and is working fine.

There are 2 callbacks back into C#

1st : The python call passes an int: AppBind.SendInt(5) and on the c# side, I get the int like this:

public void SendInt(PyObject o)
{
   int x = (int)o.AsManagedObject(typeof(int));
}

That works fine.

2nd: The Python call passes in a Python defined and created class:

class ItemTest(object):
    def __init__(self):
         self.TestInt = 1

...
    i = ItemTest()
    i.TestInt = 123
    AppBind.SendClass(i)

on the c# side, I have a companion class:

public class ItemTest
{
    public int TestInt;
}

And have this code on C# side

public void SendClass(PyObject o)
{
    ItemTest x = (ItemTest)o.AsManagedObject(typeof(ItemTest));
}

I just get the following error: System.InvalidCastException: 'cannot convert object to target type'

This is where I am stuck.

If I try calls like this:

if (o.HasAttr("TestInt"))
{
    PyObject v = o.GetAttr("TestInt");
}

then I get the following error: System.AccessViolationException: 'Attempted to read or write protected memory. This is often an indication that other memory is corrupt.'

EDIT: I've looked at IPyObjectDecoder and Codecs but saw that IPyObjectDecoder is deprecated, so I assume I shouldn't be using that

What is the correct way to convert the Python class to C#? Is manually decoding it the best way? Thanks!