HTML代码显示关键字方法

很早前写了这样一个方法,今天有空整理了一下.
功能是搜索页面的某个关键词 然后用高亮显示(搜索字母不区分大小写);
代码如下:
protected string ShowKeytext(string _string, string keystring)
       {
           string oldstring = _string;
           string newstring = _string;
           if (oldstring != null && oldstring.Length > 0)
           {
               int start = 0;
               ArrayList allkeys = new ArrayList();
               while (oldstring.IndexOf(keystring, start, System.StringComparison.CurrentCultureIgnoreCase) > -1)
               {
                   string tempstring = "";
                   tempstring = oldstring.Substring(oldstring.IndexOf(keystring, start, System.StringComparison.CurrentCultureIgnoreCase), keystring.Length);
                   if (!allkeys.Contains(tempstring))
                       allkeys.Add(tempstring);
                   start += oldstring.IndexOf(keystring, start, System.StringComparison.CurrentCultureIgnoreCase) + keystring.Length;
                   if (start >= oldstring.Length)
                       break;
               }
               foreach (string keys in allkeys)
               {
                   newstring = newstring.Replace(keys, "<span style='background-color:#FFFF00'>" + keys + "</span>");

               }
           }
           return newstring;

       }