如何从C＃“字符串”中的Rails-XML反序列化可为空的日期时间，不是有效的AllXsd值。”

Question

I am getting back XML from a Rails API, but when attempting to deserialize it I keep getting the error "The string '' is not a valid AllXsd value."

I feel that this should be really easy and trivial and that I am just missing some or other XML related attribute, but I'm not finding anything when looking on google. All examples there seem to use a string for the Date-Time value and then do internal parsing themselves to get to the actual nullable DateTime. That would work but feels messy and would probably look strange the next time someone sees it, at which point they'd probably try to replace it with a simple property and either miss the null during testing or be stuck where I am now.

Pardon the verbosity of the following example, but I've tried to reduce it to the smallest amount of related code from our actual implementation.

class Program
{
    static void Main(string[] args)
    {
        var samples = new Dictionary<string, DateTime?>{
            { // from Rails with populated datetime value
                "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n<response>\r\n  <latest-playing-at type=\"datetime\">2016-07-22T15:24:22+00:00</latest-playing-at>\r\n</response>\r\n"
                ,new DateTime(2016,07,22,17,24,22,DateTimeKind.Local)},
            { // from Rails with nil datetime value (THIS IS THE CASE I CARE ABOUT)
                "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\r\n<response>\r\n  <latest-playing-at nil=\"true\"/>\r\n</response>\r\n"
                ,(DateTime?)null},
            { // (Test) Serialized from dotnet with null value
                "<?xml version=\"1.0\" encoding=\"utf-8\"?>\r\n<response xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">\r\n  <latest-playing-at xsi:nil=\"true\" />\r\n</response>"
                    ,(DateTime?)null},
        };
        foreach (var sample in samples)
        {
            var xml = sample.Key;
            var expected = sample.Value;
            try
            {
                var testClass = DeserializeFromString<TestClass>(xml);
                Console.WriteLine("Expected / Actual : {0} / {1}", expected, testClass.LatestPlayingAt);
                Console.WriteLine(
                    DateTime.Equals(expected, testClass.LatestPlayingAt) ? "OK" : "Different" // ok to be different depening on time zones
                );
            }
            catch (Exception exc)
            {
                Console.WriteLine("Error: {0}", exc.Message);
                Console.WriteLine(exc);
            }
        }
        Console.ReadKey(true);
    }

    public static T DeserializeFromString<T>(string inputXML)
    {
        using (MemoryStream inStream = new MemoryStream(new UTF8Encoding(false).GetBytes(inputXML)))
        {
            T result = DeserializeFromStream<T>(inStream);
            return result;
        }
    }

    public static T DeserializeFromStream<T>(Stream inStream)
    {
        XmlSerializer serializer = new XmlSerializer(typeof(T));
        return (T)serializer.Deserialize(inStream);
    }
}

[XmlRoot("response")]
public class TestClass
{
    //[XmlElement("latest-playing-at")]                                                                                          // The string '' is not a valid AllXsd value.
    //[XmlElement("latest-playing-at", Type = typeof(DateTime?))]                                                                // The string '' is not a valid AllXsd value.
    //[XmlElement("latest-playing-at", Type = typeof(DateTime?), IsNullable = true)]                                             // The string '' is not a valid AllXsd value.
    //[XmlElement("latest-playing-at", Type = typeof(Nullable<DateTime>), IsNullable = true, Form = XmlSchemaForm.None)]         // The string '' is not a valid AllXsd value.
    [XmlElement("latest-playing-at", Type = typeof(Nullable<DateTime>), IsNullable = true, Form = XmlSchemaForm.Unqualified)]    // The string '' is not a valid AllXsd value.
    public DateTime? LatestPlayingAt { get; set; }
}

Answer 1

The issue is nil (as opposed to xsi:nil ) doesn't have any special meaning, it's just another attribute. The fact your XML happens to use it to mean the same thing isn't of any interest to the serialiser.

As I see it, you have two options:

1. Serialise to and from a string and handle the parsing and conversion to string yourself.
2. Pre-process the XML to replace all instances of nil with xsi:nil .

The first option is fairly self explanatory (and you mentioned it in your question). The second can be done with code like this:

var doc = XDocument.Parse(xml);

XNamespace xsi = "http://www.w3.org/2001/XMLSchema-instance";

var nilAttributes = doc.Descendants()
    .Attributes("nil")
    .Where(x => x.Value == "true");

foreach (var attribute in nilAttributes)
{
    var element = attribute.Parent;
    attribute.Remove();            
    element.Add(new XAttribute(xsi + "nil", true));
}

You can then pass the XmlReader created by doc.CreateReader() to the serialiser or get the new XML string by calling doc.ToString() .