/// <summary>
        /// This method gets called when the user uses an external identity provider to authenticate.
        /// </summary>
        /// <param name="externalUser">The external user.</param>
        /// <param name="message">The signin message.</param>
        /// <returns>
        /// The authentication result.
        /// </returns>
        public virtual Task<AuthenticateResult> AuthenticateExternalAsync(ExternalIdentity externalUser, SignInMessage message)
        {
            var query =
                from u in _users
                where
                    u.Provider == externalUser.Provider &&
                    u.ProviderId == externalUser.ProviderId
                select u;

            var user = query.SingleOrDefault();
            if (user == null)
            {
                string displayName;

                var name = externalUser.Claims.FirstOrDefault(x => x.Type == Constants.ClaimTypes.Name);
                if (name == null)
                {
                    displayName = externalUser.ProviderId;
                }
                else
                {
                    displayName = name.Value;
                }

                user = new User
                {
                    Subject = RandomString(16),
                    Provider = externalUser.Provider,
                    ProviderId = externalUser.ProviderId,
                    Username = displayName,
                    Claims = externalUser.Claims
                };
                _users.Add(user);
            }

            //var p = IdentityServerPrincipal.Create(user.Subject, GetDisplayName(user), Constants.AuthenticationMethods.External, user.Provider);
            //var result = new AuthenticateResult(p);
            //return Task.FromResult(result);
            return Task.FromResult(new AuthenticateResult("/core/account/eula", user.Subject, GetDisplayName(user)));
        }
        /// <summary>
        /// Retrieves the display name.
        /// </summary>
        /// <param name="user">The user.</param>
        /// <returns></returns>
        protected virtual string GetDisplayName(User user)
        {
            var nameClaim = user.Claims.FirstOrDefault(x => x.Type == Constants.ClaimTypes.Name);
            if (nameClaim != null)
            {
                return nameClaim.Value;
            }

            return user.Username;
        }