Web API helper extension methods for OAuth registration

RegistrationHelpers.cs

public static void RegisterOAuth(this HttpConfiguration config, FacebookOAuthClient client) {
    var existingFactory = config.RequestHandlers;

    config.RequestHandlers = (c, e, od) => {
        if (existingFactory != null)
            existingFactory(c, e, od);

        var authorizeAttribute = od.Attributes.OfType<AuthorizeAttribute>().FirstOrDefault();
        if (authorizeAttribute == null) return;
        if (od.Name == "GetCommentsOauth") {
            c.Add(new OAuthFacebookOpHandler(authorizeAttribute, client.AppId));
        }
    };

    var handlers = new List<DelegatingHandler>();
    if (config.MessageHandlerFactory != null)
        handlers.AddRange(config.MessageHandlerFactory());

    config.MessageHandlerFactory = () => {
        handlers.Add(new OAuthFacebookMessageHandler(client.AppId, client.Secret));
        return handlers;
    };
}