public static int Test_NativeArray_ArgumentFixed(IntPtr arrPtr, int multiplier)
        {
            // ArrPointer as argument does not work here!

            ArrPointer8 <int> arr = new ArrPointer8 <int>(arrPtr);

            return(arr.Sum(el => el * multiplier));;
        }
 public static int Callback(ArrPointer8 <int> arr, int multiplier) => arr.Sum(el => el * multiplier);