public async Task UpdateIdentityServerUser(string userId, bool enabled)
        {
            if (string.IsNullOrEmpty(userId))
            {
                throw new ArgumentNullException("userId");
            }
            var adminStore = new IdentityServer3AdminStore();
            var user       = await adminStore.FindIdentityServerUserByUserIdAsync(userId);

            if (user != null)
            {
                user.Enabled = enabled;
                await adminStore.UpdateIdentityServerUserAsync(user);
            }
        }