Insert a string inside string using c#

try
            {
                string _strline = "<ul>This is ul</ul><li>This is li</li><b>This is bold</b>";
                string append = "xmlns=" + "\" http://www.w3.org/1999/xhtml\"";
                string result = string.Empty;
                var arr = Regex.Matches(_strline, @"<[A-Za-z-']+>").Cast<Match>().Select(m => m.Value).ToArray();
                var openTags = arr.Distinct().ToArray();
                result = _strline;
                for (int i = 0; i < openTags.Length; i++)
                {
                    var temp = openTags[i].Replace(">", " " + append + " >");
                    result = result.Replace(openTags[i], temp);

                }
                Console.WriteLine(_strline);
                Console.WriteLine(result);
                Console.ReadKey();
            }
            catch (Exception ex)
            {
                throw ex;
            }