为什么XmlWriter.Create()失败了我的测试与Rss20FeedFormatter和SyndicationF

下面的测试通过，仅仅是因为使用了new XmlTextWriter(sw)而不是XmlWriter.Create(sw, settings) or XmlTextWriter.Create(sw):

    public void ShouldGenerateRssFeed()
    {
        //reference: [http://dotnetslackers.com/articles/aspnet/How-to-create-a-syndication-feed-for-your-website.aspx]
        var items = new List<SyndicationItem>
        {
            new SyndicationItem
            {
                Content = TextSyndicationContent.CreatePlaintextContent("This is plain test content for first item."),
                PublishDate = DateTime.Now,
                Summary = TextSyndicationContent.CreatePlaintextContent("Item summary for first item…"),
                Title = TextSyndicationContent.CreatePlaintextContent("First Item Title")
            },
            new SyndicationItem
            {
                Content = TextSyndicationContent.CreatePlaintextContent("This is plain test content for second item."),
                PublishDate = DateTime.Now,
                Summary = TextSyndicationContent.CreatePlaintextContent("Item summary for second item…"),
                Title = TextSyndicationContent.CreatePlaintextContent("Second Item Title")
            },
            new SyndicationItem
            {
                Content = TextSyndicationContent.CreateXhtmlContent("This is <strong>XHTML</strong> test content for <em>third</em> item."),
                PublishDate = DateTime.Now,
                Summary = TextSyndicationContent.CreatePlaintextContent("Item summary for third item…"),
                Title = TextSyndicationContent.CreatePlaintextContent("Third Item Title")
            }
        };
        var feed = new SyndicationFeed(items);
        Assert.IsTrue((new List<SyndicationItem>(feed.Items)).Count == 3, "The expected number of Syndication items is not here.");
        feed.Items.ForEachInEnumerable(i =>
        {
            i.Authors.Add( new SyndicationPerson
            {
                Email = "rasx@songhaysystem.com",
                Name = "Bryan Wilhite",
                Uri = "http://SonghaySystem.com"
            });
        });
        var formatter = new Rss20FeedFormatter(feed);
        var settings = new XmlWriterSettings
        {
            Encoding = Encoding.UTF8,
            Indent = true,
            IndentChars = "    "
        };
        var buffer = new StringBuilder();
        var output = string.Empty;
        using(var sw = new StringWriter(buffer))
        {
            var writer = new XmlTextWriter(sw); //XmlWriter.Create(sw, settings) or XmlTextWriter.Create(sw) fails here!
            formatter.WriteTo(writer);
            output = buffer.ToString();
            TestContext.WriteLine(output);
        }
        Assert.IsTrue(!output.Equals(string.Empty), "The expected output is not here.");
    }

顺便说一下，我在上面的示例中使用了扩展方法ForEachInEnumerable:

    /// <summary>
/// Extensions for <see cref="System.Collections.Generic.IEnumerable&lt;T&gt;"/>.
/// </summary>
public static class IEnumerableOfTExtensions
{
    /// <summary>
    /// Performs the <see cref="System.Action"/>
    /// on each item in the enumerable object.
    /// </summary>
    /// <typeparam name="TEnumerable">The type of the enumerable.</typeparam>
    /// <param name="enumerable">The enumerable.</param>
    /// <param name="action">The action.</param>
    /// <remarks>
    /// “I am philosophically opposed to providing such a method, for two reasons.
    /// …The first reason is that doing so violates the functional programming principles
    /// that all the other sequence operators are based upon. Clearly the sole purpose of a call
    /// to this method is to cause side effects.”
    /// —Eric Lippert, “foreach” vs “ForEach” [http://blogs.msdn.com/b/ericlippert/archive/2009/05/18/foreach-vs-foreach.aspx]
    /// </remarks>
    public static void ForEachInEnumerable<TEnumerable>(this IEnumerable<TEnumerable> enumerable, Action<TEnumerable> action)
    {
        foreach(var item in enumerable)
        {
            action(item);
        }
    }
}

    [TestMethod]
    public void ShouldGenerateRssFeed()
    {
        //reference: [http://dotnetslackers.com/articles/aspnet/How-to-create-a-syndication-feed-for-your-website.aspx]
        var items = new List<SyndicationItem>
        {
            new SyndicationItem
            {
                Content = TextSyndicationContent.CreatePlaintextContent("This is plain test content for first item."),
                PublishDate = DateTime.Now,
                Summary = TextSyndicationContent.CreatePlaintextContent("Item summary for first item…"),
                Title = TextSyndicationContent.CreatePlaintextContent("First Item Title")
            },
            new SyndicationItem
            {
                Content = TextSyndicationContent.CreatePlaintextContent("This is plain test content for second item."),
                PublishDate = DateTime.Now,
                Summary = TextSyndicationContent.CreatePlaintextContent("Item summary for second item…"),
                Title = TextSyndicationContent.CreatePlaintextContent("Second Item Title")
            },
            new SyndicationItem
            {
                Content = TextSyndicationContent.CreateXhtmlContent("This is <strong>XHTML</strong> test content for <em>third</em> item."),
                PublishDate = DateTime.Now,
                Summary = TextSyndicationContent.CreatePlaintextContent("Item summary for third item…"),
                Title = TextSyndicationContent.CreatePlaintextContent("Third Item Title")
            }
        };
        var feed = new SyndicationFeed(items)
        {
            Description = TextSyndicationContent.CreatePlaintextContent("My feed description."),
            Title = TextSyndicationContent.CreatePlaintextContent("My Feed")
        };
        feed.Authors.Add(new SyndicationPerson
        {
            Email = "rasx@songhaysystem.com",
            Name = "Bryan Wilhite",
            Uri = "http://SonghaySystem.com"
        });
        Assert.IsTrue((new List<SyndicationItem>(feed.Items)).Count == 3, "The expected number of Syndication items is not here.");
        feed.Items.ForEachInEnumerable(i =>
        {
            i.Authors.Add(feed.Authors.First());
        });
        var formatter = new Rss20FeedFormatter(feed);
        var settings = new XmlWriterSettings
        {
            CheckCharacters = true,
            CloseOutput = true,
            ConformanceLevel = ConformanceLevel.Document,
            Encoding = Encoding.UTF8,
            Indent =  true,
            IndentChars = "    ",
            NamespaceHandling = NamespaceHandling.OmitDuplicates,
            NewLineChars = "'r'n",
            NewLineHandling = NewLineHandling.Replace,
            NewLineOnAttributes = true,
            OmitXmlDeclaration = false
        };
        var buffer = new StringBuilder();
        var output = string.Empty;
        using(var writer = XmlWriter.Create(buffer, settings))
        {
            formatter.WriteTo(writer); // or feed.SaveAsRss20(writer);
            writer.Flush();
            writer.Close();
            output = buffer.ToString();
        }
        TestContext.WriteLine(output);
        Assert.IsTrue(!output.Equals(string.Empty), "The expected output is not here.");
    }