Keep null values in List when serializing to XML

Question

I have a list that can contain objects of different types and also nullvalues. When I serialize the class to XML, I want to keep these null values but they get automatically removed. Setting IsNullable = true in the XmlArrayItem attribute did not change anything.

I have the following simplified structure of classes:

public class MyClass
{
    [XmlArray("Items")]
    [XmlArrayItem("TypeA", typeof(A), IsNullable = true)]
    [XmlArrayItem("TypeB", typeof(B), IsNullable = true)]
    public ObservableCollection<Base> MyCollection { get; set; }
}

public class Base
{

}

public class A : Base
{

}

public class B : Base
{

}

As said before, MyCollection can contain objects of 2 different types but also null values (in my example at index 0 and 2).

This is my serialization code:

var myClass = new MyClass();
myClass.MyCollection = new ObservableCollection<Base>
{
    null,
    new A(),
    null,
    new B()
};

var stream = new FileStream("C:\\temp\\test.xml", FileMode.Create);
var serializer = new XmlSerializer(typeof (MyClass));
serializer.Serialize(stream, myClass);

I get the following XML output:

<?xml version="1.0"?>
<MyClass xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Items>
    <TypeA />
    <TypeB />
  </Items>
</MyClass>

How can I keep the null values in the list? I searched for a while but I only found solutions how to remove null-properties.

pierroz · Accepted Answer · 2014-12-09 11:43:40Z

Hmm, indeed, I haven't succeeded in making working the XmlArrayItem with the IsNullable property but if you don't mind a slightly different XML Output you can try that:

public class MyClass
    {
        [XmlArray("Items", IsNullable = true)]
        public ObservableCollection<Base> MyCollection { get; set; }
    }

    [XmlInclude(typeof(A))]
    [XmlInclude(typeof(B))]
    public class Base
    {

    }

    [XmlType("TypeA")]
    public class A : Base
    {

    }

    [XmlType("TypeB")]
    public class B : Base
    {

    }

    public static void Test()
    {
        var myClass = new MyClass() { MyCollection = new ObservableCollection<Base> { new A(), null, new B(), null } };
        var wtr = new StreamWriter("C:\\avp\\test.xml");
        var s = new XmlSerializer(typeof(MyClass));
        s.Serialize(wtr, myClass);
        wtr.Close();
    }

Then you'll get that output:

<MyClass xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"    xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Items>
    <Base xsi:type="TypeA" />  
    <Base xsi:nil="true" />
    <Base xsi:type="TypeB" />
    <Base xsi:nil="true" />
  </Items>
</MyClass>

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