C# Online Compiler | .NET Fiddle

using System;
using System.IO;
using System.Text;
using System.Windows;
using System.Xml;
using System.Xml.Serialization;

public class Program
{
	public static void Main()
	{
		ResponseBase result1 = Deserialize("<?xml version=\"1.0\"?>\r\n  <root>\r\n    <elementOne>101</elementOne>\r\n    <elementTwo>10</elementTwo>\r\n  </root>");
		ResponseBase result2 = Deserialize("<?xml version=\"1.0\"?>\r\n  <root>\r\n    <elementOne>101</elementOne>\r\n    <elementTwo>10</elementTwo>\r\n    <elementThree>10</elementThree>\r\n  </root>");
		
		PrintResult(result1);
		PrintResult(result2);
	}
	
	private static void PrintResult(ResponseBase result)
	{
	    if(result is DerivedOneClass)
		{
			DerivedOneClass derived = (DerivedOneClass)result;
			Console.WriteLine("Result is of type 'DerivedOneClass': {0}, {1}", derived.ElementOne, derived.ElementTwo);
		}
		else if(result is DerivedTwoClass)
		{
			DerivedTwoClass derived = (DerivedTwoClass)result;
			Console.WriteLine("Result is of type 'DerivedTwoClass': {0}, {1}, {2}", derived.ElementOne, derived.ElementTwo, derived.ElementThree);
		}
	}
	
	private static ResponseBase Deserialize(string xml)
	{
		MemoryStream stream = new MemoryStream(Encoding.UTF8.GetBytes(xml));
		
		
		XmlSerializer serializer = new XmlSerializer(typeof(ResponseBase));
		XmlDocument document = new XmlDocument();
		document.Load(stream);
		
		AddTypeDefinition(document);
		
		XmlReader reader = new XmlNodeReader(document);
		ResponseBase result = serializer.Deserialize(reader) as ResponseBase;
		return result;
	}
	
	private static void AddTypeDefinition(XmlDocument document)
        {
            const string xsiNamespaceUri = "http://www.w3.org/2001/XMLSchema-instance";

            XmlNode node = document.SelectSingleNode("root");
            if (node == null) return;

            var typeAttribute = node.Attributes["type", xsiNamespaceUri];
            if (typeAttribute != null) return;

            string type = "DerivedOneClass";

            XmlNodeList nodes = node.SelectNodes("//elementThree");
            if (nodes != null && nodes.Count > 0)
                type = "DerivedTwoClass";

            XmlAttribute attribute = document.CreateAttribute("xsi", "type", xsiNamespaceUri);
            attribute.Value = type;
            node.Attributes.Append(attribute);
        }

        [XmlInclude(typeof(DerivedOneClass))]
        [XmlInclude(typeof(DerivedTwoClass))]
        [XmlRoot(ElementName = "root")]
        public class ResponseBase
        {
        }

        public class DerivedOneClass: ResponseBase
        {
          [XmlElementAttribute("elementOne")]
          public string ElementOne {get; set;}
          [XmlElementAttribute("elementTwo")]
          public string ElementTwo {get; set;}
        }

        public class DerivedTwoClass: ResponseBase
        {
          [XmlElementAttribute("elementOne")]
          public string ElementOne {get; set;}
          [XmlElementAttribute("elementTwo")]
          public string ElementTwo {get; set;}
          [XmlElementAttribute("elementThree")]
          public string ElementThree {get; set;}
        }
}

 
using System;
using System.IO;
using System.Text;
using System.Windows;
using System.Xml;
using System.Xml.Serialization;
​
public class Program
{
    public static void Main()
    {
        ResponseBase result1 = Deserialize("<?xml version=\"1.0\"?>\r\n  <root>\r\n    <elementOne>101</elementOne>\r\n    <elementTwo>10</elementTwo>\r\n  </root>");
        ResponseBase result2 = Deserialize("<?xml version=\"1.0\"?>\r\n  <root>\r\n    <elementOne>101</elementOne>\r\n    <elementTwo>10</elementTwo>\r\n    <elementThree>10</elementThree>\r\n  </root>");
        
        PrintResult(result1);
        PrintResult(result2);
    }
    
    private static void PrintResult(ResponseBase result)
    {
        if(result is DerivedOneClass)
        {
            DerivedOneClass derived = (DerivedOneClass)result;
            Console.WriteLine("Result is of type 'DerivedOneClass': {0}, {1}", derived.ElementOne, derived.ElementTwo);