// -- [1] ここから ----
using Freee.OAuth.AspNetCore;
using Microsoft.AspNetCore.Authentication.Cookies;
// -- [1] ここまでを追加 ----

// ... 略 ...

namespace BasicWebApp
{
    public class Startup
    {

        // ... 略 ...

        public void ConfigureServices(IServiceCollection services)
        {
            services.Configure<CookiePolicyOptions>(options =>
            {
                // This lambda determines whether user consent for non-essential cookies is needed for a given request.
                options.CheckConsentNeeded = context => true;
                options.MinimumSameSitePolicy = SameSiteMode.None;
            });

            // -- [2] ここから ----
            services.AddAuthentication(options =>
                    {
                        options.DefaultAuthenticateScheme = CookieAuthenticationDefaults.AuthenticationScheme;
                        options.DefaultSignInScheme = CookieAuthenticationDefaults.AuthenticationScheme;
                        options.DefaultChallengeScheme = FreeeAuthenticationDefaults.AuthenticationScheme;
                    })
                    .AddCookie()
                    .AddFreee(options =>
                    {
                        options.ClientId = Configuration["Freee:ClientId"];
                        options.ClientSecret = Configuration["Freee:ClientSecret"];
                        options.SaveTokens = true;
                    });
            // -- [2] ここまでを追加 ----

            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();
            app.UseCookiePolicy();

            // -- [3] ここから ----
            app.UseAuthentication();
            // -- [3] ここまでを追加 ----

            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });
        }

using System.Threading.Tasks;

using Freee.Accounting;

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Rest;

namespace BasicWebApp.Controllers
{
    public class AccountController : Controller
    {
        [HttpGet]
        public IActionResult Login(string returnUrl = "/")
        {
            return Challenge(new AuthenticationProperties { RedirectUri = returnUrl });
        }

        public IActionResult Logout(string returnUrl = "/")
        {
            return SignOut(new AuthenticationProperties { RedirectUri = returnUrl }, CookieAuthenticationDefaults.AuthenticationScheme);
        }

        [Authorize]
        [HttpGet]
        public async Task<IActionResult> Me()
        {
            var accessToken = await HttpContext.GetTokenAsync("access_token");

            var accountingClient = new AccountingClient(new TokenCredentials(accessToken));

            var user = await accountingClient.Users.GetMeAsync(companies: true);

            var deals = await accountingClient.Deals.ListAsync(user.User.Companies[0].Id, limit: 5);

            ViewBag.Deals = deals.Deals;

            return View(user.User);
        }
    }
}