Add Google Authentication

using Microsoft.Extensions.Options;
using Umbraco.Cms.Core;
using Umbraco.Cms.Web.BackOffice.Security;

namespace MyCustomUmbracoProject.ExternalUserLogin.GoogleAuthentication;

public class GoogleBackOfficeExternalLoginProviderOptions : IConfigureNamedOptions<BackOfficeExternalLoginProviderOptions>
{
    public const string SchemeName = "OpenIdConnect";
    public void Configure(string? name, BackOfficeExternalLoginProviderOptions options)
    {
        ArgumentNullException.ThrowIfNull(name);

        if (name != Constants.Security.BackOfficeExternalAuthenticationTypePrefix + SchemeName)
        {
            return;
        }

        Configure(options);
    }

    public void Configure(BackOfficeExternalLoginProviderOptions options)
    {
        // Customize the login button
        options.ButtonStyle = "btn-danger";
        options.Icon = "icon-cloud";

        // The following options are only relevant if you
        // want to configure auto-linking on the authentication.
        options.AutoLinkOptions = new ExternalSignInAutoLinkOptions(

            // Set to true to enable auto-linking
            autoLinkExternalAccount: true,

            // [OPTIONAL]
            // Default: "Editor"
            // Specify User Group.
            defaultUserGroups: new[] { Constants.Security.EditorGroupAlias },

            // [OPTIONAL]
            // Default: The culture specified in appsettings.json.
            // Specify the default culture to create the User as.
            // It can be dynamically assigned in the OnAutoLinking callback.
            defaultCulture: null,

            // [OPTIONAL]
            // Enable the ability to link/unlink manually from within
            // the Umbraco backoffice.
            // Set this to false if you don't want the user to unlink 
            // from this external login provider.
            allowManualLinking: true
        )
        {
            // [OPTIONAL] Callback
            OnAutoLinking = (autoLinkUser, loginInfo) =>
            {
                // Customize the user before it's linked.
                // Modify the User's groups based on the Claims returned
                // in the external ogin info.
            },
            OnExternalLogin = (user, loginInfo) =>
            {
                // Customize the User before it is saved whenever they have
                // logged in with the external provider.
                // Sync the Users name based on the Claims returned
                // in the external login info

                // Returns a boolean indicating if sign-in should continue or not.
                return true;
            }
        };

        // [OPTIONAL]
        // Disable the ability for users to login with a username/password.
        // If set to true, it will disable username/password login
        // even if there are other external login providers installed.
        options.DenyLocalLogin = false;

        // [OPTIONAL]
        // Choose to automatically redirect to the external login provider
        // effectively removing the login button.
        options.AutoRedirectLoginToExternalProvider = false;
    }
}

using Umbraco.Cms.Core.DependencyInjection;
using Umbraco.Extensions;
using Microsoft.Extensions.DependencyInjection;

namespace MyCustomUmbracoProject.ExternalUserLogin.GoogleAuthentication;

public static class GoogleAuthenticationExtensions
{
    public static IUmbracoBuilder AddGoogleAuthentication(this IUmbracoBuilder builder)
    {
        // Register ProviderBackOfficeExternalLoginProviderOptions here rather than require it in startup
        builder.Services.ConfigureOptions<GoogleBackOfficeExternalLoginProviderOptions>();

        builder.AddBackOfficeExternalLogins(logins =>
        {
            logins.AddBackOfficeLogin(
                backOfficeAuthenticationBuilder =>
                {

                    // The scheme must be set with this method to work for the back office
                    var schemeName = backOfficeAuthenticationBuilder.SchemeForBackOffice(GoogleBackOfficeExternalLoginProviderOptions.SchemeName);

                    ArgumentNullException.ThrowIfNull(schemeName);

                    backOfficeAuthenticationBuilder.AddGoogle(
                        schemeName,
                        options =>
                        {
                            // Callback path: Represents the URL to which the browser should be redirected to.
                            // The default value is '/signin-google'.
                            // The value here should match what you have configured in you external login provider.
                            // The value needs to be unique.
                            options.CallbackPath = "/umbraco-google-signin";
                            options.ClientId = "YOURCLIENTID"; // Replace with your client id generated while creating OAuth client ID
                            options.ClientSecret = "YOURCLIENTSECRET"; // Replace with your client secret generated while creating OAuth client ID
                        });
                });
        });
        return builder;
    }
}

using MyCustomUmbracoProject.ExternalUserLogin.GoogleAuthentication;

public void ConfigureServices(IServiceCollection services)
{
    services.AddUmbraco(_env, _config)
        .AddBackOffice()
        .AddWebsite()
        .AddComposers()

        // Register your new static extension class
        .AddGoogleAuthentication()

        .Build();
}