单元测试两个C＃列表

Question

我有一个接收对象列表集合的方法。想法是该方法将检查传递给它的每个对象中的特定字段CoverArt，如果CoverArt属性为null，空字符串或空白，它将设置为默认值。如果CoverArt属性不为null，空字符串或空格，则它仅返回其中已经存在的任何字符串。

//checks a list of stories for cover art and sets cover art to a default if it is null
   public string CheckIfCoverArtIsNullForStoryList(List<Story> stories)
   {
    foreach (var story in stories)
    {
        if (story.CoverArt == null || story.CoverArt == "" || story.CoverArt == " ")
    {
    return story.CoverArt = "default-book.png";
   }
   return story.CoverArt;
}

            return "";
        }

我要做的是创建一个单元测试，以测试此方法是否有效，但是我很难比较两个列表。我的代码有什么问题？

[TestMethod]
        [Description("Tests list of stories for null cover art")]
        public void TestingStoryListCoverArtMethodWithValidInput()
        {
            List<Story> StoryListActual = new List<Story>()
            {
                new Story()
                {
                    CoverArt = "Lord of the Rings.png"
                },
                new Story()
                {
                    CoverArt = "Johnny Appleseed.jpeg"
                },
                new Story()
                {
                    CoverArt = "The Great Gatsby.jpg"
                },
                new Story()
                {
                    CoverArt = "Happy Gilmore.gif"
                }
            };

            List<Story> CoverArtExpected = new List<Story>()
            {
                new Story()
                {
                    CoverArt = "Lord of the Rings.png"
                },
                new Story()
                {
                    CoverArt = "Johnny Appleseed.jpeg"
                },
                new Story()
                {
                    CoverArt = "The Great Gatsby.jpg"
                },
                new Story()
                {
                    CoverArt = "Happy Gilmore.gif"
                }
            };

            helper.CheckIfCoverArtIsNullForStoryList(StoryListActual);

            for (var i = 0; i < StoryListActual.Count; i++)
            {
                for (var j = 0; j < CoverArtExpected.Count; j++)
                {
                    Assert.AreEqual(StoryListActual[i].CoverArt, CoverArtExpected[j].CoverArt);
                }
            }
        }

我的单元测试失败，并显示此消息。