using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var strValue = "a/ new string, with some@ values&";

        Console.WriteLine(strValue.Replace("/@&", '_'));

    }

}

​

public static class Extensions {

    public static string Replace(this string source, string blacklist, char letter) => 

        new string(source.Select(c => blacklist.Contains(c) ? letter : c).ToArray());

}