how to pass array of objects from c# to c++ using Dllimport?

Question

I am working on project that required to pass array of objects from c# to c++ by reference and when I pass one object by reference it return by the new value successfully but when I pass an array it return with the old value so what is wrong with my code?

C++ code

extern "C" __declspec(dllexport) int solve(Cell*(&xx)[5][5]) {
xx[0][0]->side = 55;
return xx[0][0]->side;
}

and C# code

internal static class UnsafeMethods
{
[DllImport(@"E:\Cs\4th Year\HPC\Parallel_calculations\Debug\Parallel_calculations.dll", EntryPoint = "solve", CallingConvention = CallingConvention.Cdecl)]
public static extern int solve([MarshalAs(UnmanagedType.SafeArray)] ref Cell[,] x);
}

 Cell[,] arr = new Cell[5, 5];
        for (int i = 0; i < arr.GetLength(0); i++)
        {
            for (int y = 0; y < arr.GetLength(1); y++)
            {
                arr[i, y] = new Cell(0, 0, 0, false, false, false, 50);
            }
        }
        arr[0, 0].side = 4;
        int x = UnsafeMethods.solve(ref arr);
        Console.WriteLine(x + " " + arr[0, 0].side);

x is with new value but arr[0,0].side return with old value