Startup.cs

using Adyen;
using Adyen.Service.Checkout;
using Adyen.Util;
using adyen_dotnet_giving_example.Options;
using adyen_dotnet_giving_example.Services;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Options;
using System;
using System.Net.Http;
using System.Threading;

namespace adyen_dotnet_giving_example
{
    public class Startup
    {
        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.
        public void ConfigureServices(IServiceCollection services)
        {
            // Configure your keys using the Options pattern.
            // This will auto-retrieve/configure your keys from your environmental variables (ADYEN_CLIENT_KEY, ADYEN_API_KEY, ADYEN_MERCHANT_ACCOUNT, ADYEN_HMAC_KEY).
            services.Configure<AdyenOptions>(
                options =>
                {
                    options.ADYEN_CLIENT_KEY = Configuration[nameof(AdyenOptions.ADYEN_CLIENT_KEY)];
                    options.ADYEN_API_KEY = Configuration[nameof(AdyenOptions.ADYEN_API_KEY)];
                    options.ADYEN_MERCHANT_ACCOUNT = Configuration[nameof(AdyenOptions.ADYEN_MERCHANT_ACCOUNT)];
                    options.ADYEN_HMAC_KEY = Configuration[nameof(AdyenOptions.ADYEN_HMAC_KEY)]; 
                }
            );

            // Register your controllers.
            services.AddControllersWithViews();
            services.AddControllers().AddNewtonsoftJson();
            
            // Used to store the `PspReference` and `DonationToken` for Adyen Giving (donations).
            services.AddSession(options =>
            {
                options.Cookie.Name = "Adyen.Giving.Session";
                options.IdleTimeout = TimeSpan.FromMinutes(30);
                options.Cookie.IsEssential = false;
            });

            services.AddHttpContextAccessor()
                .AddTransient<IUrlService, UrlService>();

            // Register Adyen.Client as singleton.
            string httpClientName = "HttpClientName";

            services.AddSingleton((IServiceProvider provider) =>
            {
                AdyenOptions options = provider.GetRequiredService<IOptions<AdyenOptions>>().Value;
                Config config = new Config()
                {
                    // Get your `API Key` from AdyenOptions using the Options pattern.
                    XApiKey = options.ADYEN_API_KEY,
                    // Test environment.
                    Environment = Adyen.Model.Environment.Test,
                };
                return new Client(config, provider.GetRequiredService<IHttpClientFactory>(), httpClientName);
            });

            // Register named HttpClient.
            services.AddHttpClient(httpClientName)
            .ConfigurePrimaryHttpMessageHandler((IServiceProvider provider) =>
            {
                return new SocketsHttpHandler()
                {
                    PooledConnectionLifetime = TimeSpan.FromMinutes(1)
                };
            }).SetHandlerLifetime(Timeout.InfiniteTimeSpan);

            services.AddSingleton<IDonationsService, DonationsService>();
            services.AddSingleton<IPaymentsService, PaymentsService>(); // Used to be called "Checkout.cs" in Adyen .NET 9.x.x and below, see https://github.com/Adyen/adyen-dotnet-api-library/blob/9.2.1/Adyen/Service/Checkout.cs.
            services.AddSingleton<HmacValidator>();
        }

        // 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("/error");
            }

            app.UseStaticFiles();
            app.UseRouting();

            app.UseAuthorization();

            app.UseSession();

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