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  `GET`で`HttpClient`を作り直してるからじゃないですか？

  あとクッキーを使いたいだけなら、`UseCookies = true`でいいんじゃないですかね。

 

  using System;

  using System.Diagnostics;

  using System.Linq;

  using System.Net.Http;

  using System.Text;

  using System.Threading.Tasks;

 

  namespace Questions251616

  {

  static class Program

  {

  static async Task Main()

  {

  Console.Write("username >>>");

  var user = Console.ReadLine();

 

  Console.Write("password >>>");

  var password = Console.ReadLine();

 

  var obj = new API();

  var result = await obj.LoginAsync(user, password);

  Console.WriteLine(result);

  Console.WriteLine();

 

  var self = await obj.SelfAsync();

  Console.WriteLine(self);

 

  Console.ReadKey();

  }

  }

 

  public class API

  {

  // 長いｗ

  private static readonly HttpClient client = new HttpClient(new HttpClientHandler() { UseCookies = true, })

  {

  BaseAddress = new Uri("https://judgeapi.u-aizu.ac.jp/"),

  };

 

  public async Task<string> LoginAsync(string id, string pass)

  {

  var json = $@"{{""id"":""{id}"",""password"":""{pass}""}}";

  Debug.WriteLine(json);

 

  var content = new StringContent(json, Encoding.UTF8, "application/json");

  var response = await client.PostAsync("session", content);

  Debug.WriteLine($"content ->> {content}");

 

  if(response.Headers.Contains("Set-Cookie"))

  {

  var cookie = response.Headers.GetValues("Set-Cookie").First();

  Debug.WriteLine($"cookie --> {cookie}\n");

  }

 

  return await response.Content.ReadAsStringAsync();

  }

 

  public Task<string> SelfAsync() => GetAsync("self");

 

  public async Task<string> GetAsync(string uri)

  {

  var response = await client.GetAsync(uri);

  return await response.Content.ReadAsStringAsync();

  }

  }

  }

  ```

 

  [.NET(Framework)のHttpClientの取り扱いには要注意という話 - Qiita](https://qiita.com/nskhara/items/b7c31d60531ffbe29537)

  を見ると`Static`も`UseCookies = true`も要注意なのですが^^;