Upgrade to .NET Core 3.1

Last week I had the task of upgrading a project written in .NET Core 2.1 to .NET Core 3.1. I'll be posting solutions to the errors I came across. But first, I'll show important sections of my Startup.cs and Program.cs:

I'll start with Program.cs:


        public static void Main(string[] args)
        {
            var host = CreateHostBuilder(args).Build();
            host.Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .UseLamar()
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();
                }).ConfigureLogging(builder =>
                {                    
                    ...

                    builder.AddApplicationInsights();

                });

On to the file Startup.cs

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

        public IConfiguration Configuration { get; }

        public void ConfigureContainer(ServiceRegistry services)
        {
            services.Configure<CookiePolicyOptions>(options =>
            {
...
            });

            services.AddAuthentication(AzureADDefaults.AuthenticationScheme)
                .AddAzureAD(options => Con

            services.AddMediatR();
            services.AddAutoMapper();
            services.AddSession();

...
            services.AddControllersWithViews(options =>
            {
                var policy = new AuthorizationPolicyBuilder()
                    .RequireAuthenticatedUser()
                    .Build();

                options.Filters.Add(typeof(AzureAuthorizationFilter));
...
                options.UseModelBinding();
            });
            services.AddRazorPages().SetCompatibilityVersion(CompatibilityVersion.Latest).AddNewtonsoftJson(options => options.SerializerSettings.ContractResolver = new DefaultContractResolver())
                .AddTypedRouting()
                .AddNewtonsoftJson(x => x.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore);
            services.AddAuthorization();
            services.Configure<RazorViewEngineOptions>(options => {      
...
            });

            services.AddSignalR();

            services.Scan(scan =>
            {
                scan.TheCallingAssembly();
...
                scan.WithDefaultConventions();
            });
            services.For<IPrincipal>().Use(x => Thread.CurrentPrincipal);
...
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                app.UseHsts();
            }

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

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

                endpoints.MapHub<NotificationHub>("/pushHub");
            });
        }

    }