How to serialize to XML abstract class in c#

Question

I keep getting error

The type Domains.Logic.Class1+ChildBody was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically.

The problem is i already use XmlInclude attribute.

        [XmlInclude(typeof(ChildBody))]        
        public abstract class BaseBody
        {
        }
        public class ChildBody : BaseBody
        {
            public string Message {get;set;}
        }
        [XmlRoot(ElementName = "Envelope", Namespace = "http://schemas.xmlsoap.org/soap/envelope/")]
        [Serializable]
        public class EnvelopeModel
        {
            [XmlElement(ElementName = "Body")]
            public BaseBody Body { get; set; }
        }
        public void Serialize()
        {
            using (TextWriter writer = new StreamWriter(@"g:\test\xml.xml", false, System.Text.Encoding.UTF8))
            {

                XmlSerializer serializer = new XmlSerializer(typeof(EnvelopeModel));
                serializer.Serialize(writer, new EnvelopeModel { Body = new ChildBody { Message = "example"} });
            }
        }