Importing Nim modules from C#

ImportNim.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;

namespace nim_sharp
{
    [StructLayout(LayoutKind.Sequential)]
    public unsafe struct NimSring
    {
        public Int32 Len;
        public Int32 Capacity;
        public char Text;
    }

    [StructLayout(LayoutKind.Sequential)]
    public unsafe struct NimSeq
    {
        public Int32 Len;
        public Int32 Capacity;
        public byte Data;
    }

    [StructLayout(LayoutKind.Sequential)]
    public unsafe struct NimType_ext
    {
        public Int32 x;
        public Int32 y;
        public NimSring* s;
    }

    public struct NimType
    {
        public int x;
        public int y;
        public string s;
    }

    class Program
    {
        [DllImport("nimlib.dll", CallingConvention = CallingConvention.Cdecl)]
        public static unsafe extern void populateArraySruct(int length, NimSeq** ArrayStructs);
 
        public static unsafe List<NimType> getResultsFromNim(int ArrL)
        {
            List<NimType> convertedResults = new List<NimType>(ArrL);
            
            NimSeq* nimResults = null;
            populateArraySruct(ArrL, &nimResults);
            
            NimType_ext* currentItem = (NimType_ext*) &(nimResults->Data);
            for (int i = 0; i < ArrL; i++, currentItem++)
            {
                convertedResults.Add(new NimType() {x = currentItem->x, y = currentItem->y, s = new string(&(currentItem->s->Text))});
            }
            return convertedResults;
        }

        static void Main(string[] args)
        {
            var list = getResultsFromNim(10);

            foreach (var elem in list) {
                Console.WriteLine("x: {0}, y: {1}, s: {2}", elem.x, elem.y, elem.s);
            }
        }
    }
}

nimlib.nim

# compile with nim c -app:lib nimlib.nim
# place next to the C# executable together with a compiled copy of nimrtl.dll

type
  NimTypeSeq = seq[NimType]
  NimType = object {.packed.}
    x: int
    y: int
    s: string
 
proc populateArraySruct(outputLength: int, output: var NimTypeSeq) {.exportc,dynlib.} =
  setupForeignThreadGc()

  output.newSeq(outputLength)
  
  for i in 0 .. <outputLength:
    output[i] = NimType(x: i, y: i*2, s: $i)
    

and yet another clue which contrasts my opinion on the relation with nimrtl.dll is when it's removed from the folder it has no effect, so it has nothing to do with nimrtl.dll as if it does not use it or maybe when you compiled against nimrtl.dll it allready did what it needs to do with nimrtl.dll so now the nimlib.dll allready have what it needs ...could be possible i really don't know the relations between them so it's for someone with the knowledge to figure it out.

uninstalled nim 32/64 version which was a 64bit setup

installed full package of 32bit nim setup
now it does work encoding is wrong shows string as????
but members of struct are now aligned and i guess you are running 32bit too
and your project i have downloaded is 32bit am i right or am i right?

s = new string(&(currentItem->s->Text))});
this will throw an error bad pointer in 64 bit if it is clear for you as to why...

Yes, my builds were 32-bit and I'm using a Nim compiler compiled straight from the devel branch. On Windows, I also use the older 32-bit mingw distribution.

Basically, these problems are not very hard to trouble-shoot, but you have to be a bit more seasoned C/C++ developer. You can always examine the C code produced by Nim (placed in the nimcache folder) and you can use the "Memory" debug window in Visual Studio to see the actual data written by the Nim dll. From there, it's just a matter of comparing your conceptual model expectations with the reality you see in the debugger.

the only problem i have left as i mention in former post, as we only pass numeric char now it does work but encoding is wrong shows string as??? is this an issue or simple solution ? try concat $i as s= "abc" & $i

Nim strings are usually stored in UTF-8, although the language doesn't require that. In C#, the encoding is UCS-2 (UTF-16). Take a look at the following stackoverflow answer that features some conversion code:
http://stackoverflow.com/questions/10773440/conversion-in-net-native-utf-8-managed-string

StringFromNativeUtf8 is the one i think, i will take a time to test both ways to see if that is the solution, as this has to be the one, i will report to you as soon as i find the time to play with it again, thanks i think that concludes the subject of the string passing from nim to c# which was not a simple task for me, as soon as i will confront this, and do some more tests i will be more familiar and it will be simpler from that point, i'll let you know. thanks a lot as ususall !