Startup.cs

using System;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.HttpOverrides;
using Microsoft.AspNetCore.SpaServices.ReactDevelopmentServer;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace mst
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            var connection = Configuration.GetConnectionString("DefaultConnection");
            services.AddDbContext<DatabaseContext>(options =>
                options.UseMySql(
                    connection,
                    new MySqlServerVersion(new Version(8, 0, 20))
                )
            );

            services.AddControllersWithViews();

            // In production, the React files will be served from this directory
            services.AddSpaStaticFiles(configuration => { configuration.RootPath = "ClientApp/build"; });

            // Authentication
            services.AddAuthentication(options =>
                {
                    options.DefaultScheme = CookieAuthenticationDefaults.AuthenticationScheme;
                })
                .AddCookie()
                .AddGoogle(options =>
                {
                    options.ClientId = Configuration["Google:ClientId"];
                    options.ClientSecret = Configuration["Google:ClientSecret"];

                    options.CallbackPath = "/Show/VoteOAuth/signin-google";
                })
                .AddFacebook(options =>
                {
                    options.AppId = Configuration["Facebook:AppId"];
                    options.AppSecret = Configuration["Facebook:AppSecret"];

                    options.CallbackPath = "/Show/VoteOAuth/signin-facebook";
                })
                .AddVkontakte(options =>
                {
                    options.ClientId = Configuration["Vk:ClientId"];
                    options.ClientSecret = Configuration["Vk:ClientSecret"];

                    options.CallbackPath = "/Show/VoteOAuth/signin-vk";
                });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseForwardedHeaders(new ForwardedHeadersOptions
            {
                ForwardedHeaders = ForwardedHeaders.XForwardedProto
            });

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/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.UseSpaStaticFiles();

            app.UseRouting();

            app.UseAuthentication();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    "default",
                    "{controller}/{action=Index}/{id?}");
            });

            app.UseSpa(spa =>
            {
                spa.Options.SourcePath = "ClientApp";

                if (env.IsDevelopment())
                    spa.UseReactDevelopmentServer("start");
            });
        }
    }
}