HatenaBlogDownloader.cs

using System;
using System.IO;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
using System.Xml;

namespace HatenaBlogDownloader
{
    class Program
    {
        static async Task Main(string[] args)
        {
            Console.Write("ID: ");
            var id = Console.ReadLine();
            Console.Write("Blog ID: ");
            var blogId = Console.ReadLine();
            Console.Write("API Key: ");
            var apiKey = Console.ReadLine();

            using (var client = new HttpClient())
            {
                client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
                    "Basic", Convert.ToBase64String(Encoding.UTF8.GetBytes(id + ":" + apiKey)));

                var requestUri = "https://blog.hatena.ne.jp/" + id + "/" + blogId + "/atom/entry";

                for (var i = 1; requestUri != null; i++)
                {
                    var outputName = $"{i:D03}.xml";
                    Console.WriteLine(requestUri + " -> " + outputName);

                    using (var res = await client.GetStreamAsync(requestUri))
                    using (var fs = new FileStream(outputName, FileMode.Create, FileAccess.ReadWrite))
                    {
                        res.CopyTo(fs);
                        res.Dispose();

                        fs.Seek(0, SeekOrigin.Begin);
                        requestUri = null;

                        using (var reader = XmlReader.Create(fs))
                        {
                            const string xmlns = "http://www.w3.org/2005/Atom";

                            do
                            {
                                if (!reader.Read()) throw new Exception();
                            } while (reader.NodeType != XmlNodeType.Element);

                            reader.ReadStartElement("feed", xmlns);

                            while (reader.ReadToNextSibling("link", xmlns))
                            {
                                if (reader.GetAttribute("rel") == "next")
                                {
                                    requestUri = reader.GetAttribute("href");
                                    break;
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}

HttpLinkChecker.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Xml.Linq;
using AngleSharp.Dom;
using AngleSharp.Extensions;
using AngleSharp.Parser.Html;

namespace HttpLinkChecker
{
    class Program
    {
        private static readonly XNamespace s_atomNs = "http://www.w3.org/2005/Atom";
        private static readonly XNamespace s_hatenaNs = "http://www.hatena.ne.jp/info/xmlns#";
        private static readonly HtmlParser s_parser = new HtmlParser();

        public static void Main(string[] args)
        {
            foreach (var path in Directory.GetFiles(".", "*.xml"))
            {
                foreach (var entry in XElement.Load(path).Elements(s_atomNs + "entry"))
                {
                    var found = false;

                    foreach (var src in FindHttpSrc(entry.Element(s_hatenaNs + "formatted-content").Value))
                    {
                        if (!found)
                        {
                            found = true;
                            Console.WriteLine(entry.Element(s_atomNs + "title").Value);
                            Console.WriteLine(
                                entry.Elements(s_atomNs + "link")
                                    .Single(x => ((string)x.Attribute("rel")) == "alternate")
                                    .Attribute("href")
                                    .Value
                            );
                            Console.WriteLine();
                        }

                        Console.WriteLine(src);
                    }

                    if (found)
                    {
                        Console.WriteLine();
                        Console.WriteLine();
                    }
                }
            }
        }

        private static IEnumerable<string> FindHttpSrc(string html)
        {
            var document = s_parser.Parse(html);
            return document.Descendents<IElement>()
                .Select(x => x.GetAttribute("src"))
                .Where(x => x != null && x.StartsWith("http://", StringComparison.OrdinalIgnoreCase));
        }
    }
}