using System;

using System.Runtime.InteropServices;

public class Program

{

    public static void Main()

    {

        const string constStr = "000";

        Mutate(constStr);

​

        var nonConst = "000";

        Console.WriteLine(nonConst);

​

        void Mutate(string str)

        {

            var memory = MemoryMarshal.AsMemory(str.AsMemory());

            memory.Span[0] = '4';

            memory.Span[2] = '4';

        }

    }

}