NimString in C#

export.nim

# variant 2) convert the Nim string to a manually managed piece of memory

type
  Exported = objec
    len: int
    data: ptr char # this has to be freed with Nim's `dealloc` proc

proc marshal(x: string): Exported =
  result.len = x.len
  result.data = create(char, x.len + 1)
  copyMem(result.data, cstring(x), x.len + 1)


// but here i am complitly lost what do i use as the buffer: the Exported above?

proc GetPacksPtrNim(parSze: int, PackArrINOUT: var DataPackArr){.exportc,dynlib.} =
  PackArrINOUT.newSeq(parSze)
  var dummy:string ="abcdefghij"
  for i, curDataPack in PackArrINOUT.mpairs:
                          # here i generate value by adding the iterator i to the buffer so i will know for sure it's new values on each element
    curDataPack = DataPack(buffer:dummy & $i, intVal: uint32 i)
#and it's the simulation of putting any kind of unknown length string from a file or database...
#that is why i bother with the trailing char(i + int8'0') 

NimString.cs

// variant 1) directly exposing the Nim string type to C#
...
  [StructLayout(LayoutKind.Sequential)]
  public struct NimString
  {
      public uint Len;
      public uint Capacity;
      public LPStr Text;
  }
...

  [StructLayout(LayoutKind.Sequential)]
  public struct TestC
  {
      public uint Id;
      public NimString Str;
  }
  
// variant 2) converting strings to manually managed memory before passing them to C#
// my Version so far

        [StructLayout(LayoutKind.Sequential)]
        public struct NimString
        {
            public uint Len;
            public uint Capacity;
            public char* Text;
        }
        [StructLayout(LayoutKind.Sequential)]
        public struct TestNimStr
        {
            public uint Id;
            public NimString Str;
        }
        
        //iterop Signature
        [DllImport(@"G:\RobDev\Documents\Visual Studio 2010\Projects\CPP_Interaction\Nim\x64\testnimatom.dll", CallingConvention = CallingConvention.Cdecl), SuppressUnmanagedCodeSecurity]
        public static extern void GetPacksPtrNim(int size, TestNimStr** outPackArr);

        //this is how i return from Nim
        public static unsafe List<TestNimStr> PopulateLstPackCharNim(int ArrL)
        {
            TestNimStr* PackUArrOut = null;
            List<TestNimStr> RtPackLst = new List<TestNimStr>(ArrL);
            GetPacksPtrNim(ArrL, &PackUArrOut);
            TestNimStr* CurrentPack = PackUArrOut;
            for (int i = 0; i < ArrL; i++, CurrentPack++)
            {
               // here it was simple ..where Strval is IntPtr later i marahsll it... so it actually also two steps 
               //with the original DataPack-IntPtr member
                RtPackLst.Add(new TestNimStr() { StrVal = CurrentPack->StrVal, IntVal = CurrentPack->IntVal });
                string accessedElementStr = Marshal.PtrToStringAnsi(RtPackLst.element(number).Strval);
               // but how do i access here so there's actually no extra steps in your approach i realise while writing
                RtPackLst.Add(new TestNimStr() { Str = CurrentPack->Str, Id = CurrentPack->IntVal });
               // Which is NimString struct
               var testAccessStructAsString = RtPackLst.element(number).Str;
               //so actually: as string ---v   method likes it             |------v (is a char*)
               string buildTheString = new string(RtPackLst.element(number).Str.Text) 
            }

            return RtPackLst;
        }

this was actually simple(C# side), i will now try to work on the complete implementation of your approach on the nim side