using System;
using System.IO;
using System.Xml.Serialization;

namespace XmlSerializationApplication
{
    [Serializable, XmlRoot("class")]
    public class MyClass
    {
        [XmlAttribute("attribute")]
        public string Attribute { get; set; }

        [XmlElement("element")]
        public string Element { get; set; }
    }

    public class Program
    {
        static void Main()
        {
            using (var stream = File.Create("MyClass.xml"))
            {
                var target = new MyClass { Attribute = "La-la-la", Element = "Tu-tu-tu" };
                var xml = new XmlSerializer(target.GetType());

                xml.Serialize(stream, target);
            }
        }
    }
}

<?xml version="1.0"?>
<class xmlns:xsd="http://www.w3.org/2001/XMLSchema"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       attribute="La-la-la">
    <element>Tu-tu-tu</element>
</class>