Question

I have a class containing an array I wish to serialize with XmlSerializer:

[XmlArray("properties")]
[XmlArrayItem("property", IsNullable = true)]
public List<Property> Properties { get; set; }

Property is a class containing an attribute and some XmlText:

[XmlAttribute("name")]
public string Name { get; set; }

[XmlText]
public string Value { get; set; }

The problem is that when Value is null, it serializes as an empty string:

<property name="foo" />

rather than a null. I’m looking for the value to either be omitted entirely, or look like this:

<property name="foo" xsi:nil="true" />

Is it possible to null out an element in a list based on its XmlText value? I’m really trying to avoid custom serialization, but perhaps some other serialization framework would be better in this case?

Answer 1

Use the IsNullable=true in the XmlArrayItemAttribute class. For an example.

[XmlRoot("Root")]
public class Root
{
    [XmlArrayItem("Element", IsNullable = true)]
    public string[] Elements { get; set; }
}

Some sample code in Visual Studion 2012 and .Net 4.5:

using System.Xml.Serialization;

...

// Test object
Root root;
root = new Root();
root.Elements = new string[] { null, "abc" };

using(MemoryStream stream = new MemoryStream())
{
    XmlSerializer xmlSerializer = new XmlSerializer(typeof(Root));
    xmlSerializer.Serialize(stream, root);

    Console.WriteLine(new string(Encoding.UTF8.GetChars(stream.GetBuffer())));
}

The output is (line breaks added for clarity):

<?xml version="1.0"?>
<Root 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Element>
    <string xsi:nil="true" />
    <string>abc</string>
  </Element>
</Root>

And with a complex type (also in .Net 4.5 on Visual Studio 2012):

    public class MyProperty
    {
        public string Foo { get; set; }
    }

    [XmlRoot("Root")]
    public class Root
    {
        [XmlArrayItem("Element", IsNullable = true)]
        public MyProperty[] Elements { get; set; }
    }

    ,,,

    Root root;
    root = new Root();
    root.Elements = new MyProperty[] { null, new MyProperty{ Foo = "bar" } };

    // Other code is as above

Using the same code above produces:

<?xml version="1.0"?>
<Root 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Elements>
    <Element xsi:nil="true" />
    <Element>
      <Foo>bar</Foo>
    </Element>
  </Elements>
</Root>

Also remember that the type must be a reference type (not a struct, for example) to write out xsi:nil=true.