help with pinvoke and variant return type

[fhanggi]

Could anyone please help me write a pinvoke statement for this function?

[id(0x60030004)]
HRESULT AFAccountPayment(
[in] VARIANT coForm,
[in, out, optional] BSTR* billlist,
[out, retval] VARIANT* );

I have tried a few things but it's not working. I don't know how to code the VARIAN* retval properly.
Mostly i get an error: PInvoke restriction: cannot return variants.

Here is what i have tried so far.
<DllImport("c:\windows\system32\nctk.dll")> _
Public Shared Function AFAccountPayment(ByVal coForm As Object, Optional ByRef billList As String = "") As Object
End Function

<DllImport("c:\windows\system32\nctk.dll")> _
Public Shared Function AFAccountPayment(<Out(), MarshalAs(UnmanagedType.Struct)> ByVal coForm As Object, <[In](), [Out](), MarshalAs(UnmanagedType.VBByRefStr)> Optional ByRef billList As String = Nothing) As Object
End Function

Thank you for any help.

[Paul.Martin]

The best thing I can think of suggesting (without knowing what the VARIANT* will actually be pointing to and if it will be marshallable) is changing the semantics of the PInvoke call. So you could try something like:

<DllImport("c:\windows\system32\nctk.dll")> _
Public Shared Function AFAccountPayment(<Out(), MarshalAs(UnmanagedType.Struct)> ByVal coForm As Object, <[In](), [Out](), MarshalAs(UnmanagedType.VBByRefStr)> Optional ByRef billList As String = Nothing, <Out(), MarshalAs(UnmanagedType.Struct)> ByRef varRet As Object) As IntPtr
End Function