C# ITypeInfo.GetContainingTypeLib fails when passe

So I have experimented on calling ITypeInfo on a VBA Class instance and whilst it looks promising I wanted to see if I could get a reference to its containing project, an analogue to a type library. I thought ITypeInfo.GetContainingTypeLib might be useful but it throws an exception indicating VBA won't co-operate. Anyone got any ideas about how VBA maybe doing things differently from the standard COM specification?

The C# Class library code is here. Register for COM interop and set COMVisible(true) in AssemblyInfo.cs to make it accessible from VBA. VBA client code given below.

using System;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;


namespace TypeLibraryInspector
{
    [ComImport()]
    [Guid("00020400-0000-0000-C000-000000000046")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IDispatch
    {
        [PreserveSig]
        int GetTypeInfoCount(out int Count);

        [PreserveSig]
        int GetTypeInfo
            (
                [MarshalAs(UnmanagedType.U4)] int iTInfo,
                [MarshalAs(UnmanagedType.U4)] int lcid,
                out System.Runtime.InteropServices.ComTypes.ITypeInfo typeInfo
            );


        //void GetTypeInfo(int typeInfoIndex, int lcid, [MarshalAs(UnmanagedType.CustomMarshaler,
        //        MarshalTypeRef = typeof(System.Runtime.InteropServices.CustomMarshalers.TypeToTypeInfoMarshaler))] out Type typeInfo);

        //void GetTypeInfo(int typeInfoIndex, int lcid,  out IntPtr piTypeInfo);


        [PreserveSig]
        int GetIDsOfNames
            (
                ref Guid riid,
                [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPWStr)]
                string[] rgsNames,
                int cNames,
                int lcid,
                [MarshalAs(UnmanagedType.LPArray)] int[] rgDispId
            );

        [PreserveSig]
        int Invoke
            (
                int dispIdMember,
                ref Guid riid,
                uint lcid,
                ushort wFlags,
                ref System.Runtime.InteropServices.ComTypes.DISPPARAMS pDispParams,
                out object pVarResult,
                ref System.Runtime.InteropServices.ComTypes.EXCEPINFO pExcepInfo,
                IntPtr[] pArgErr
            );
    }


    public interface IInspector
    {
        void InspectThisObject(object vbaClassInstance);
    }

    [ClassInterface(ClassInterfaceType.None)]
    [ComDefaultInterface(typeof(IInspector))]

    public class Inspector : IInspector
    {
        private const int S_OK = 0; //From WinError.h
        private const int LOCALE_SYSTEM_DEFAULT = 2 << 10; //From WinNT.h == 2048 == 0x800



        void IInspector.InspectThisObject(object vbaClassInstance)
        {
            //https://limbioliong.wordpress.com/2011/10/18/obtain-type-information-of-idispatch-based-com-objects-from-managed-code/
            IDispatch pDispatch = (IDispatch)vbaClassInstance;

            ITypeInfo piTypeInfo;
            pDispatch.GetTypeInfo(0, LOCALE_SYSTEM_DEFAULT, out piTypeInfo);

            string s1; string s2; string s3;
            int i1;
            piTypeInfo.GetDocumentation(-1, out s1, out s2, out i1, out s3);
            //s1 = "Class1" good
            //s2 = null     shame

            ITypeLib piTypeLib;
            int pIndex;

            piTypeInfo.GetContainingTypeLib(out piTypeLib, out pIndex); // <-- throws Exception 0x800A88C1

        }
    }
}

So some client VBA is here

Sub Test()

    Dim oInspector As TypeLibraryInspector.Inspector
    Set oInspector = New TypeLibraryInspector.Inspector

    Dim oClass1 As Class1
    Set oClass1 = New Class1

    oInspector.InspectThisObject oClass1

End Sub

where Class1 can be any class, I have two empty functions in mine but I do not think that relevant.

I have asked equivalent C++ question.

if the dll is com exposed it has to be registered with regsvr32

Also u might need to GAC the .net assembly