Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Identity;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.AspNetCore.Identity.UI.Services;
using Northwind.Models;
using Northwind.Services;
using System;
using Microsoft.AspNetCore.DataProtection;
using Microsoft.Extensions.Options;
using Microsoft.Extensions.Logging;

namespace Northwind
{
    public class Startup
    {
        // this class needs the connection info stored in the 
        // appsettings.json config file - that's a dependency
        // with dependency injection we expose the config file to this class
        public IConfiguration Configuration { get; }
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<AppIdentityDbContext>(options => options.UseSqlServer(Configuration["Data:NWIdentity:ConnectionString"]));
            
            services.AddIdentity<AppUser, IdentityRole>(opts =>
            {
                opts.User.RequireUniqueEmail = true;
                opts.Password.RequiredLength = 6;
                opts.Password.RequireNonAlphanumeric = false;
                opts.Password.RequireLowercase = false;
                opts.Password.RequireUppercase = false;
                opts.Password.RequireDigit = false;
                opts.Password.RequiredUniqueChars = 1;
                opts.SignIn.RequireConfirmedEmail = true;
                opts.Tokens.ProviderMap.Add("CustomEmailConfirmation", 
                    new TokenProviderDescriptor(typeof(CustomEmailConfirmationTokenProvider<AppUser>)));
                opts.Tokens.EmailConfirmationTokenProvider = "CustomEmailConfirmation";
            }).AddEntityFrameworkStores<AppIdentityDbContext>().AddDefaultTokenProviders();

            services.AddTransient<CustomEmailConfirmationTokenProvider<AppUser>>();

            // this is where we use the config info for our connection string
            services.AddDbContext<NorthwindContext>(options => options.UseSqlServer(Configuration["Data:Northwind:ConnectionString"]));

            // since we created an interface for our repository, we must map the 
            // interface to the concrete class to ensure that when an INorthwindRepository
            // is requested, a new instance of EFNorthwindRepository is returned
            services.AddTransient<INorthwindRepository, EFNorthwindRepository>();

            // Change all data protection token lifespans to 4 hours
            services.Configure<DataProtectionTokenProviderOptions>(o => o.TokenLifespan = TimeSpan.FromHours(4));

            // requires
            // using Microsoft.AspNetCore.Identity.UI.Services;
            // using Northwind.Services
            services.AddTransient<IEmailSender, EmailSender>();
            services.Configure<AuthMessageSenderOptions>(Configuration.GetSection("Data:SendGrid"));

            services.ConfigureApplicationCookie(o => {
                o.ExpireTimeSpan = TimeSpan.FromDays(10);
                o.SlidingExpiration = true;
            });

            services.AddMvc();
        }

        // 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();
            }

            // Make the authentication service available to the application (Order matters here)
            app.UseAuthentication();
            app.UseStaticFiles();
            app.UseMvcWithDefaultRoute();
        }
    }

    public class CustomEmailConfirmationTokenProvider<TUser>
                                           : DataProtectorTokenProvider<TUser> where TUser : class
    {
        public CustomEmailConfirmationTokenProvider(IDataProtectionProvider dataProtectionProvider,
            IOptions<EmailConfirmationTokenProviderOptions> options,
            ILogger<DataProtectorTokenProvider<TUser>> logger)
                                              : base(dataProtectionProvider, options)
        {

        }
    }

    public class EmailConfirmationTokenProviderOptions : DataProtectionTokenProviderOptions
    {
        public EmailConfirmationTokenProviderOptions()
        {
            Name = "EmailDataProtectorTokenProvider";
            TokenLifespan = TimeSpan.FromHours(4);
        }
    }
}