static class StringExtensions
    {
        public static string BracketSelf(this string instance)
        {
            return "[" + instance + "]";
        }
    }

    string s = "Text";
    Console.WriteLine(s.BracketSelf());
    // this should output [Text]

    string s = null;
    Console.WriteLine(s.BracketSelf());
    // this should output []
    Console.WriteLine(s.ToLower());
    // this should throw a NullReferenceException

    static class StringExtensions
    {
        public static string BracketSelf(this string instance)
        {
            if (instance == null) throw new NullReferenceException();
            return "[" + instance + "]";
        }
    }

    public static bool IsNull(this object instance) {
        return instance == null;
    }

    string s = null;
    Console.WriteLine(s.IsNull());
    // this should output true
    s = "Some Text";
    Console.WriteLine(s.IsNull());
    // this should output false