Custom external authentication provider

CustomExternalAuthenticationProvider.cs

public class CustomGoogleAuthenticationProvider : SitefinityGoogleAuthenticationProvider
{
    public override Task Authenticated(GoogleOAuth2AuthenticatedContext context)
    {
        var isIdentityValid = YourCustomCheck();
        if (!isIdentityValid)
        {
            context.Identity = null;
            return Task.FromResult(0);
        }

        return base.Authenticated(context);
    }
}


public class CustomAuthenticationProvidersInitializer : AuthenticationProvidersInitializer
{
    public override Dictionary<string, Action<IAppBuilder, string, AuthenticationProviderElement>> GetAdditionalIdentityProviders()
    {
        var providers = base.GetAdditionalIdentityProviders();

        providers["Google"] =
               (IAppBuilder app, string signInAsType, AuthenticationProviderElement config) =>
               {
                    var googleConfig = config as GoogleAuthenticationProviderElement;

                    if (googleConfig != null)
                    {
                        var opt = new GoogleOAuth2AuthenticationOptions
                        {
                            AuthenticationType = googleConfig.Name,
                            Caption = googleConfig.Title,
                            SignInAsAuthenticationType = signInAsType,
                            ClientId = googleConfig.AppId,
                            ClientSecret = googleConfig.AppSecret,
                            Provider = new CustomGoogleAuthenticationProvider()
                        };

                        app.UseGoogleAuthentication(opt);
                    }
               };


        return providers;
    }
}


public class Global : System.Web.HttpApplication
{
    protected void Application_Start(object sender, EventArgs e)
    {
        SystemManager.ApplicationStart += SystemManager_ApplicationStart;
    }

    private void SystemManager_ApplicationStart(object sender, EventArgs e)
    {
        ObjectFactory.Container.RegisterType<AuthenticationProvidersInitializer, CustomAuthenticationProvidersInitializer>(new ContainerControlledLifetimeManager());
    }
}