Serialize a C# class to XML with attributes and a single value for the class

Try using [XmlText]:

public class Title
{
  [XmlAttribute("id")]
  public int Id { get; set; }

  [XmlText]
  public string Value { get; set; }
}

Here’s what I get (but I didn’t spend a lot of time tweaking the XmlWriter, so you get a bunch of noise in the way of namespaces, etc.:

<?xml version="1.0" encoding="utf-16"?>
<Title xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:xsd="http://www.w3.org/2001/XMLSchema"
       id="123"
       >Grand Poobah</Title>