Use XmlWriter to create your XML, it does the necessary escaping for
you. If you only want to create a text then use XmlWriterSettings with
ConformanceLevel.Fragment e.g.

      XmlWriterSettings settings = new XmlWriterSettings();
      settings.ConformanceLevel = ConformanceLevel.Fragment;
      StringWriter writer = new StringWriter();
      using (XmlWriter xmlWriter = XmlWriter.Create(writer, settings))
      {
        xmlWriter.WriteString("a < b && b < a");
      }
      Console.WriteLine(writer.ToString());

writes "a &lt; b &amp;&amp; b &lt; a".

The best approach is to use an API which lets you build up the XML
document just by specifying the strings, and it will do the escaping
for you - XmlWriter, XmlDocument etc.