kowill/Program.cs

## Program.cs
using System.IO;
using System.Linq;
using System.Reflection;
using System.Xml;
using System.Xml.Linq;

namespace DtdSample
{
    class Program
    {
        static void Main(string[] args)
        {
            string path = Path.Combine(Path.GetDirectoryName(Assembly.GetEntryAssembly().Location), @"Resources\Test.xml");
            //.NET Framework 4.5.2からはコメントアウト行の記述ではダメ
            //using (var reader = XmlReader.Create(path, new XmlReaderSettings { DtdProcessing = DtdProcessing.Parse }))
            using (var reader = XmlReader.Create(path, new XmlReaderSettings { DtdProcessing = DtdProcessing.Parse, XmlResolver = new XmlUrlResolver() }))
            {
                XDocument srcTree = XDocument.Load(reader);
                var data = srcTree.Descendants("Hoge")
                .Select(x =>
                {
                    var item = new Hoge();
                    item.Category = int.Parse(x.Element("Category").Value);
                    item.Title = x.Element("Title").Value;
                    item.Body = x.Element("Body").Value;
                    return item;
                })
                .ToList();
            }
        }
    }

    class Hoge
    {
        public int Category { get; set; }
        public string Title { get; set; }
        public string Body { get; set; }
    }
}

## Sub1.txt
サブ１
てすとーーーーーーーーーーーーーーーーーー

## Test.xml
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE Samples[
  <!ELEMENT Samples (Hoge+)>
  <!ELEMENT Hoge (Category,Title,Body)>
  <!ELEMENT Category (#PCDATA)>
  <!ELEMENT Title (#PCDATA)>
  <!ELEMENT Body (#PCDATA)>
  <!ENTITY Sub1 SYSTEM "Body\Sub1.txt">
  <!ENTITY Sub2 SYSTEM "Body\Sub2.txt">
  <!ENTITY Sub3 SYSTEM "Body\Sub3.txt">
]>
<Samples>
  <Hoge>
    <Category>0</Category>
    <Title>タイトル１</Title>
    <Body>&Sub1;</Body>
  </Hoge>

  <Hoge>
    <Category>1</Category>
    <Title>タイトル２</Title>
    <Body>&Sub1;</Body>
  </Hoge>

  <Hoge>
    <Category>2</Category>
    <Title>タイトル３</Title>
    <Body>&Sub1;</Body>
  </Hoge>
</Samples>
	using System.IO;
	using System.Linq;
	using System.Reflection;
	using System.Xml;
	using System.Xml.Linq;

	namespace DtdSample
	{
	class Program
	{
	static void Main(string[] args)
	{
	string path = Path.Combine(Path.GetDirectoryName(Assembly.GetEntryAssembly().Location), @"Resources\Test.xml");
	//.NET Framework 4.5.2からはコメントアウト行の記述ではダメ
	//using (var reader = XmlReader.Create(path, new XmlReaderSettings { DtdProcessing = DtdProcessing.Parse }))
	using (var reader = XmlReader.Create(path, new XmlReaderSettings { DtdProcessing = DtdProcessing.Parse, XmlResolver = new XmlUrlResolver() }))
	{
	XDocument srcTree = XDocument.Load(reader);
	var data = srcTree.Descendants("Hoge")
	.Select(x =>
	{
	var item = new Hoge();
	item.Category = int.Parse(x.Element("Category").Value);
	item.Title = x.Element("Title").Value;
	item.Body = x.Element("Body").Value;
	return item;
	})
	.ToList();
	}
	}
	}

	class Hoge
	{
	public int Category { get; set; }
	public string Title { get; set; }
	public string Body { get; set; }
	}
	}
	サブ１
	てすとーーーーーーーーーーーーーーーーーー
	<?xml version="1.0" encoding="utf-8" ?>
	<!DOCTYPE Samples[
	<!ELEMENT Samples (Hoge+)>
	<!ELEMENT Hoge (Category,Title,Body)>
	<!ELEMENT Category (#PCDATA)>
	<!ELEMENT Title (#PCDATA)>
	<!ELEMENT Body (#PCDATA)>
	<!ENTITY Sub1 SYSTEM "Body\Sub1.txt">
	<!ENTITY Sub2 SYSTEM "Body\Sub2.txt">
	<!ENTITY Sub3 SYSTEM "Body\Sub3.txt">
	]>
	<Samples>
	<Hoge>
	<Category>0</Category>
	<Title>タイトル１</Title>
	<Body>&Sub1;</Body>
	</Hoge>

	<Hoge>
	<Category>1</Category>
	<Title>タイトル２</Title>
	<Body>&Sub1;</Body>
	</Hoge>

	<Hoge>
	<Category>2</Category>
	<Title>タイトル３</Title>
	<Body>&Sub1;</Body>
	</Hoge>
	</Samples>