//引入命名空间
using System;
using System.Web;
using System.IO;

class MyNewClass
{
    public static void Main()
    {
        Console.WriteLine("Enter a string having '&', '<', '>' or '\"' in it: ");
        string myString = Console.ReadLine();

        // Encode the string.
        string myEncodedString = HttpUtility.HtmlEncode(myString);

        Console.WriteLine($"HTML Encoded string is: {myEncodedString}");
        StringWriter myWriter = new StringWriter();

        // Decode the encoded string.
        HttpUtility.HtmlDecode(myEncodedString, myWriter);

        string myDecodedString = myWriter.ToString();
        Console.Write($"Decoded string of the above encoded string is: {myDecodedString}");
    }
}