Can one have Python send a variable-length string array to C#?

Question

This is the reverse/complement of this question.

I am using Unmanaged Exports and ctypes, and want to pass a variable-length list of strings from Python to C#, and get a string back.

I try four C# + Python variants - none succeeds. Can anyone get the job done?

C#

    /* no ref, convert to list */
    [DllExport("combineWords1", CallingConvention = CallingConvention.Cdecl)]
    [return: MarshalAs(UnmanagedType.AnsiBStr)]
    public static string CombineWords1(object obj)
    {
        var l = (List<string>) obj;
        return string.Join(",", l.ToArray());
    }

    /* no ref, convert to array */
    [DllExport("combineWords2", CallingConvention = CallingConvention.Cdecl)]
    [return: MarshalAs(UnmanagedType.AnsiBStr)]
    public static string CombineWords2(object obj)
    {
        var l = (string[]) obj;
        return string.Join(",", l);
    }

    /* ref, convert to list */
    [DllExport("combineWords3", CallingConvention = CallingConvention.Cdecl)]
    public static void CombineWords3(ref object obj)
    {
        var l = (List<string>)obj;
        obj = string.Join(",", l.ToArray());
    }

    /* ref, convert to array */
    [DllExport("combineWords4", CallingConvention = CallingConvention.Cdecl)]
    public static void CombineWords4(ref object obj)
    {
        var l = (string[]) obj;
        obj = string.Join(",", l.ToArray());
    }

Python

import ctypes
from comtypes.automation import VARIANT

dll = ctypes.cdll.LoadLibrary("<...>")
l = ["Hello", "world"]

# 1 
dll.combineWords1.argtypes = [ctypes.POINTER(VARIANT)]
dll.combineWords1.restype = ctypes.c_char_p
obj = VARIANT(l)
dll.combineWords1(obj)
#WindowsError: [Error -532462766] Windows Error 0xE0434352

# 2
dll.combineWords2.argtypes = [ctypes.POINTER(VARIANT)]
dll.combineWords2.restype = ctypes.c_char_p
obj = VARIANT(l)
dll.combineWords2(obj)
# WindowsError: [Error -532462766] Windows Error 0xE0434352

# 3
dll.combineWords3.argtypes = [ctypes.POINTER(VARIANT)]
obj = VARIANT(l)
dll.combineWords3(obj)
#WindowsError: [Error -532462766] Windows Error 0xE0434352

# 4
dll.combineWords4.argtypes = [ctypes.POINTER(VARIANT)]
obj = VARIANT(l)
dll.combineWords4(obj)
#WindowsError: [Error -532462766] Windows Error 0xE0434352

Simon Mourier · Accepted Answer · 2018-02-12 06:49:51Z

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"])
print(dll.combinewords(v))

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

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