Attempt at a Google Apps OpenID client. Running into a difficult to debug problem where no useful info is returned as to why it fails.

AuthConfig.cs

using System.Collections.Generic;
using Microsoft.Web.WebPages.OAuth;

namespace MvcOauth1
{
    public static class AuthConfig
    {
        public static void RegisterAuth()
        {
            // To let users of this site log in using their accounts from other sites such as Microsoft, Facebook, and Twitter,
            // you must update this site. For more information visit http://go.microsoft.com/fwlink/?LinkID=252166

            //OAuthWebSecurity.RegisterMicrosoftClient(
            //    clientId: "",
            //    clientSecret: "");

            //OAuthWebSecurity.RegisterTwitterClient(
            //    consumerKey: "",
            //    consumerSecret: "");

            //OAuthWebSecurity.RegisterFacebookClient(
            //    appId: "",
            //    appSecret: "");

            OAuthWebSecurity.RegisterClient(new GoogleAppsOpenIdClient("example.com"), "Google Apps Example",
                                            new Dictionary<string, object>());
        }
    }
}

GoogleAppsOpenIdClient.cs

using System;
using System.Collections.Specialized;
using DotNetOpenAuth.OpenId;
using DotNetOpenAuth.OpenId.Extensions.AttributeExchange;
using DotNetOpenAuth.OpenId.Messages;
using DotNetOpenAuth.OpenId.RelyingParty;
using DotNetOpenAuth.AspNet.Clients;
using System.Collections.Generic;

    /// <summary>
    /// Represents Google OpenID client.
    /// 
    /// </summary>
    public sealed class GoogleAppsOpenIdClient : OpenIdClient
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="T:DotNetOpenAuth.AspNet.Clients.GoogleOpenIdClient"/> class.
        /// 
        /// </summary>
        public GoogleAppsOpenIdClient(string domain)
            : base(domain, Identifier.Parse(string.Format("https://www.google.com/accounts/o8/site-xrds?ns=2&hd={0}", domain)))
        {
        }

        /// <summary>
        /// Gets the extra data obtained from the response message when authentication is successful.
        /// 
        /// </summary>
        /// <param name="response">The response message.
        ///             </param>
        /// <returns>
        /// A dictionary of profile data; or null if no data is available.
        /// </returns>
        protected override Dictionary<string, string> GetExtraData(IAuthenticationResponse response)
        {
            var extension = response.GetExtension<FetchResponse>();
            if (extension == null)
                return null;
            var dictionary = new Dictionary<string, string>();
            AddItemIfNotEmpty(dictionary, "email", extension.GetAttributeValue("http://axschema.org/contact/email"));
            AddItemIfNotEmpty(dictionary, "country", extension.GetAttributeValue("http://axschema.org/contact/country/home"));
            AddItemIfNotEmpty(dictionary, "firstName", extension.GetAttributeValue("http://axschema.org/namePerson/first"));
            AddItemIfNotEmpty(dictionary, "lastName", extension.GetAttributeValue("http://axschema.org/namePerson/last"));
            return dictionary;
        }

        /// <summary>
        /// Called just before the authentication request is sent to service provider.
        /// 
        /// </summary>
        /// <param name="request">The request.
        ///             </param>
        protected override void OnBeforeSendingAuthenticationRequest(IAuthenticationRequest request)
        {
            FetchRequest fetchRequest = new FetchRequest();
            AXUtilities.AddRequired((ICollection<AttributeRequest>)fetchRequest.Attributes, "http://axschema.org/contact/email");
            AXUtilities.AddOptional((ICollection<AttributeRequest>)fetchRequest.Attributes, "http://axschema.org/contact/country/home");
            AXUtilities.AddOptional((ICollection<AttributeRequest>)fetchRequest.Attributes, "http://axschema.org/namePerson/first");
            AXUtilities.AddOptional((ICollection<AttributeRequest>)fetchRequest.Attributes, "http://axschema.org/namePerson/last");
            request.AddExtension((IOpenIdMessageExtension)fetchRequest);
        }

        protected static void AddItemIfNotEmpty(Dictionary<string, string> dictionary, string key, string value)
        {
            if (key == null)
            {
                throw new ArgumentNullException("key");
            }

            if (!string.IsNullOrEmpty(value))
            {
                dictionary[key] = value;
            }
        }
    }