EqualTest.cs

using System;

namespace EqualTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var ohdear = new
            {
                // https://www.fileformat.info/info/unicode/char/fffd/index.htm
                // Unicode replacement character
                UNICODE = StringComparer.InvariantCulture.Equals("", "\uFFFD"),
                //https://www.fileformat.info/info/unicode/char/00df/index.htm
                // Latin small s guess it is turning it to German letter?
                IS = "\u00DF".StartsWith("ss", StringComparison.InvariantCulture),
                // Normal
                THE = "ss".StartsWith("s", StringComparison.InvariantCulture),
                // Not exactly consistent
                DEVIL = "\u00DF".StartsWith("s", StringComparison.InvariantCulture)
            };
            Console.WriteLine($"UNICODE {ohdear.UNICODE}");
            Console.WriteLine($"IS {ohdear.IS}");
            Console.WriteLine($"THE {ohdear.THE}");
            Console.WriteLine($"DEVIL {ohdear.DEVIL}");
        }
    }
}

// Program Output
/*
UNICODE True
IS True
THE True
DEVIL False
*/