See the question and my original answer on StackOverflow

You can declare the C# side like this:

[DllExport("combinewords", CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.LPWStr)] // matches python's c_wchar_p
public static string CombineWords(object obj) // object matches python's VARIANT
    var array = (object[])obj; // if obj is an array, it will always be an array of object
    return string.Join(", ", array);

And the python side like this:

import ctypes
from ctypes import *
from comtypes.automation import VARIANT

dll = ctypes.cdll.LoadLibrary("exported") # the dll's name
dll.combinewords.argtypes = [VARIANT] # python VARIANT = C# object
dll.combinewords.restype = ctypes.c_wchar_p

# create a VARIANT from a python array
v = VARIANT(["hello", "world"])

Note: I have used unicode declaration wich is better. Ansi is a thing of the past.