C# XmlSerializer de-serialize abstract class without knowing concrete class

Question

I would like to deserialize xml where the root class is an Abstract class but the root node in the xml has a value of the XML root attribute in the concrete class. The key is to do this without specifying the concrete class to deserialize in the XmlSerializer constructor.

Example:

MessageType.cs

[Serializable]
[XmlInclude(typeof(Entity))]
public abstract class MessageType
{
}

Entity.cs

[Serializable]
[XmlRoot("Entity", IsNullable = false)]
public class Entity : MessageType
{
    private string data;

    public string Data
    {
        get { return data; }
        set { data = value; }
    }
}

Program.cs

var entity = new Entity
{
    Data = "test"
};

var xs = new XmlSerializer(typeof(Entity));

using var stream = new System.IO.StringWriter();

xs.Serialize(stream, entity);
var xml = stream.ToString();

using var stringReader = new System.IO.StringReader(xml);

xs = new XmlSerializer(typeof(MessageType));
var obj = serializer.Deserialize(stringReader);

This code results in

Unhandled exception. System.InvalidOperationException: There is an error in XML document (2, 2).
 ---> System.InvalidOperationException: <Entity xmlns=''> was not expected.
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationReaderMessageType.Read4_MessageType()
   --- End of inner exception stack trace ---
   at System.Xml.Serialization.XmlSerializer.Deserialize(XmlReader xmlReader, String encodingStyle, XmlDeserializationEvents events)
   at System.Xml.Serialization.XmlSerializer.Deserialize(TextReader textReader)

The xml produced by the serialization in the code above looks like:

<?xml version="1.0" encoding="utf-16"?>
<Entity xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Data>test</Data>
</Entity>

Couple Extra Notes:

1. In the real case, the xml message comes from another application that I have 0 control over.
2. The c# classes come from an XSD so I'm unable to change those either
3. There are a lot of concrete class implementations of the MessageType class and with xml deserialization seeming to be a slow process, looping over each possible type and seeing if it can deserialize is not ideal
4. I know there are a lot of similar questions on stackoverflow but have been unable to find one that answers my exact question
5. I understand if the received xml looked like

   <?xml version="1.0" encoding="utf-16"?>
   <MessageType xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
       xmlns:xsd="http://www.w3.org/2001/XMLSchema" xsi:type="Entity">
       <Data>test</Data>
   </MessageType>
   

   that it would work, but refer to note #1.

Answer 1

You can get the name of the root node with the XmlReader. Then you can get the type by name and create the serializer accordingly.

using var xmlReader = XmlReader.Create(stream);
xmlReader.MoveToContent();
var rootNodeName = xmlReader.Name;

var implementationType = Type.GetType(rootNodeName);
if (implementationType != null)
{
    var serializer = new XmlSerializer(implementationType);
    var obj = serializer.Deserialize(xmlReader);
}
else
{
    // unknown type
}