Setting up dependency injection in ASP.NET WebForms (example using SimpleInjector)

ContainerServiceProvider.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace MyWebformApp
{
    public class ContainerServiceProvider : IServiceProvider, System.Web.Hosting.IRegisteredObject
    {
        public ContainerServiceProvider(IServiceProvider next, Func<Type, object> resolverFunc)
        {
            _next = next;
            _resolver = resolverFunc;
            System.Web.Hosting.HostingEnvironment.RegisterObject(this);
        }

        private readonly IServiceProvider _next;
        private Func<Type, object> _resolver;

        private readonly System.Collections.Concurrent.ConcurrentDictionary<Type, bool> _typesCannotResolve
            = new System.Collections.Concurrent.ConcurrentDictionary<Type, bool>();

        private const int TypesCannontResolveCacheCap = 100000;

        public object GetService(Type serviceType)
        {
            if (_typesCannotResolve.ContainsKey(serviceType))
            {
                return DefaultCreateInstance(serviceType);
            }

            object result = null;
            try
            {
                result = _resolver(serviceType);
            }
            catch (Exception)
            {
                // Ignore and continue
            }

            if (result != null) return result;

            result = _next?.GetService(serviceType);

            if (result != null) return result;

            result = DefaultCreateInstance(serviceType);

            if (result == null) return result;

            // Cache it
            if (_typesCannotResolve.Count < TypesCannontResolveCacheCap)
            {
                _typesCannotResolve.TryAdd(serviceType, true);
            }

            return result;
        }

        public void Stop(bool immediate)
        {
            System.Web.Hosting.HostingEnvironment.UnregisterObject(this);
        }

        protected virtual object DefaultCreateInstance(Type type)
        {
            return Activator.CreateInstance(
                type,
                System.Reflection.BindingFlags.Instance
                | System.Reflection.BindingFlags.NonPublic
                | System.Reflection.BindingFlags.Public
                | System.Reflection.BindingFlags.CreateInstance,
                null,
                null,
                null);
        }

    }
}

Default.aspx.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace MyWebformApp
{
    public partial class Default : System.Web.UI.Page
    {
        public Default(IMyService dep)
        {
            _dependency = dep;
        }

        private IMyService _dependency;

        protected void Page_Load(object sender, EventArgs e)
        {
            // ...
        }

    }
}

Global.asax.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.SessionState;

namespace MyWebformApp
{
    public class Global : System.Web.HttpApplication
    {
        public static SimpleInjector.Container Container => new SimpleInjector.Container();

        private static object _webObjectActivatorLock = new object();

        protected void Application_Start(object sender, EventArgs e)
        {
            Container.Register<IMyService, MyService>();
            Container.Verify();

            // lock it to avoid registering ContainerServiceProvider multiple times
            lock (_webObjectActivatorLock)
            {
                HttpRuntime.WebObjectActivator = new ContainerServiceProvider(
                    HttpRuntime.WebObjectActivator,
                    serviceType => Container.GetInstance(serviceType));
            }
        }

    }
}

Web.config

<?xml version="1.0"?>
<configuration>
  <!-- ... -->
  <system.web>
    <!-- ... -->
    <compilation targetFramework="4.7.2"/>
    <httpRuntime targetFramework="4.7.2"/>
    <!-- ... -->
  </system.web>
  <!-- ... -->
</configuration>