Question

I have read many posts regarding deserialization of nullable fields but have not run across the following scenario:

1. Serialize an object with a nullable field that contains a value (“nil” attribute is not added to the node because it contains a value).
2. Remove the value from the nullable field in the xml (this happens via client-side processing).
3. Deserialize the xml.

Step 3 throws an error because the serializer does not treat the empty value of the nullable field as a null value (because “nil=true” is not specified). It instead tries to convert the value to the field’s data type (ex: Guid), which fails resulting in an error message that varies depending on the field’s data type.

In the case of a Guid the error message is:

    System.InvalidOperationException: There is an error in XML document ([line number], [column number]). ---> System.FormatException: Unrecognized Guid format.

I should note that the serialization / deserialization methods we use are framework methods that use generics.

I’m looking for an elegant and generic solution. The only feasible, generic solution I can think of is the following:

1. Convert the xml to an XDocument.
2. Use (less than desired) reflection to get all of the properties of the object that are reference types.
3. Add “nil=true” attribute to all nodes whose name is found in the list from #2 and has an empty value.
4. Use recursion to process each reference type in #2.

Note: Simply adding “nil=true” to all nodes that have an empty value will not work because the serializer will throw an error for value types that cannot be null.

[Edit] Code examples:

Sample data class

    public class DummyData
    {
        public Guid? NullableGuid { get; set; }
    }

Xml sent to client

    <DummyData>
    <NullableGuid>052ec82c-7322-4745-9ac1-20cc4e0f142d</NullableGuid>
    </DummyData>

Xml returned from client (error)

    <DummyData>
    <NullableGuid></NullableGuid>
    </DummyData>

Xml returned from client (desired result)

    <DummyData>
        <NullableGuid p2:nil="true" xmlns:p2="http://www.w3.org/2001/XMLSchema-instance"></NullableGuid>
    </DummyData>

Answer 1

Here is the solution I came up with that pretty closely resembles my plan of attack described in the original question.

Disclaimer: It is not short and most likely does not cover every deserialization scenario but seems to get the job done.

    public static T FromXml<T>(string xml)
    {
       string convertedXml = AddNilAttributesToNullableTypesWithNullValues(typeof(T), xml);
       var reader = new StringReader(convertedXml);
       var serializer = new XmlSerializer(typeof (T));
       var data = (T) serializer.Deserialize(reader);
       reader.Close();
       return data;
    }

    private static string AddNilAttributesToNullableTypesWithNullValues(Type type, string xml)
    {
        string result;

        if (!string.IsNullOrWhiteSpace(xml))
        {
            XDocument doc = XDocument.Parse(xml);

            if (doc.Root != null)
                AddNilAttributesToNullableTypesWithNullValues(doc.Root, type);

            result = doc.ToString();
        }
        else
            result = xml;

        return result;
    }

    private static void AddNilAttributesToNullableTypesWithNullValues(XElement element, Type type)
      {
         if (type == null)
            throw new ArgumentNullException("type");

         if (element == null)
            throw new ArgumentNullException("element");

         //If this type can be null and it does not have a value, add or update nil attribute
         //with a value of true.
         if (IsReferenceOrNullableType(type) && string.IsNullOrEmpty(element.Value))
         {
            XAttribute existingNilAttribute = element.Attributes().FirstOrDefault(a => a.Name.LocalName == NIL_ATTRIBUTE_NAME);

            if (existingNilAttribute == null)
               element.Add(NilAttribute);
            else
               existingNilAttribute.SetValue(true);
         }
         else
         {
            //Process all of the objects' properties that have a corresponding child element.
            foreach (PropertyInfo property in type.GetProperties())
            {
               string elementName = GetElementNameByPropertyInfo(property);

               foreach (XElement childElement in element.Elements().Where(e =>
                  e.Name.LocalName.Equals(elementName)))
               {
                  AddNilAttributesToNullableTypesWithNullValues(childElement, property.PropertyType);
               }
            }

            //For generic IEnumerable types that have elements that correspond to the enumerated type,
            //process the each element.
            if (IsGenericEnumerable(type))
            {
               Type enumeratedType = GetEnumeratedType(type);

               if (enumeratedType != null)
               {
                  IEnumerable<XElement> enumeratedElements = element.Elements().Where(e =>
                     e.Name.LocalName.Equals(enumeratedType.Name));

                  foreach (XElement enumerableElement in enumeratedElements)
                     AddNilAttributesToNullableTypesWithNullValues(enumerableElement, enumeratedType);
               }
            }
         }
      }

      private static string GetElementNameByPropertyInfo(PropertyInfo property)
      {
         string overrideElementName = property.GetCustomAttributes(true).OfType<XmlElementAttribute>().Select(xmlElementAttribute => 
            xmlElementAttribute.ElementName).FirstOrDefault();
         return overrideElementName ?? property.Name;
      }

      private static Type GetEnumeratedType(Type type)
      {
         Type enumerableType = null;

         Type[] types = type.GetGenericArguments();

         if (types.Length == 1)
            enumerableType = types[0];

         return enumerableType;
      }

      public static bool IsGenericEnumerable(Type type)
      {
         return type.IsGenericType && type.GetInterfaces().Any(i => 
            i.IsGenericType && i.GetGenericTypeDefinition() == typeof(IEnumerable<>));
      }

      private static bool IsReferenceOrNullableType(Type type)
      {
         return !type.IsValueType || Nullable.GetUnderlyingType(type) != null;
      }

      private const string NIL_ATTRIBUTE_NAME = "nil";
      private const string XML_SCHEMA_NAMESPACE = "http://www.w3.org/2001/XMLSchema-instance";

      private static XAttribute NilAttribute
      {
         get
         {
             if (_nilAttribute == null)
             {
                 XNamespace xmlSchemaNamespace = XNamespace.Get(XML_SCHEMA_NAMESPACE);
                 _nilAttribute = new XAttribute(xmlSchemaNamespace + NIL_ATTRIBUTE_NAME, true);
         }

        return _nilAttribute;
     }
  }