在unittest调试中获取异步回调值

我得到了UnitTest方法,它用回调调用方法

    [Test]
    public void GetUserAsyncTest()
    {
        User result;
        _restTest.GetUserAsync((user) =>
                                   {
                                       result = user;
                                   });

       Assert.AreEqual("xy", result.Email);
    }

 /// <summary>
    /// Retrieve the User details for the currently authenticated User
    /// </summary>
    /// <param name="callback">Method to call upon successful completion</param>
    public void GetUserAsync(Action<User> callback)

我如何测试它并从回调中获取值?目前我的结果总是空的,这是合乎逻辑的。