Xsd.exe generated class doesn't serialize mandatory elements when null value

Question

In the following XSD all elements are mandatory:

<?xml version="1.0" encoding="utf-16"?>
<xs:schema xmlns="http://TestNamespace" xmlns:b="http://schemas.microsoft.com/BizTalk/2003" targetNamespace="http://TestNamespace" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="Test">
    <xs:complexType>
      <xs:sequence>
        <xs:element minOccurs="1" maxOccurs="1" name="Id" type="xs:int" />
        <xs:element minOccurs="1" maxOccurs="1" name="EMail" type="xs:string" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

However, when I serialize an instance of the xsd.exe generated class where EMail == null, the resulting XML is invalid according to the schema, because the EMail element is missing altogether.

<?xml version="1.0"?>
<Test xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://TestNamespace">
  <Id xmlns="">2</Id>
</Test>

Why is that? Is there any way to prevent it?

What are you really asking? The serializer does the only logical thing since you *did* pass a null. Are you asking why the serializer doesn't also validate the result? Do you want to prevent the serialization of invalid classes? Validate the classes before serialization? — Panagiotis Kanavos, Apr 24 '15 at 07:23
well, since I generated the class with a tool I'd expect the results to be valid automatically, i.e. in this case the serializer should add an empty element - so my question is how to achieve it? — metalheart, Apr 24 '15 at 07:29

score 0 · Answer 1 · edited May 23 '17 at 10:24

In your comments you mention that you want an empty value to be used as a default for a missing Email element.

This is a schema-level problem, not related to the serializer or the proxy generator.

Such behaviour should be defined in the schema, not left to the serializer. Unfortunately, XML Schema doesn't allow default element values because it's far too complex to guess what the default element's structure would be. Does it have required elements itself? Arrays? What about choice elements?

XML Schema does allow Default or fixed attribute values are allowed though, using the default and fixed attributes.

One solution is to create an instance of what you consider a valid empty Email object and use it instead of null values.

static readonly EmptyEmail=new Email();
...
test.Email=test.Email??EmptyEmail;

In fact, this is one case of the Null Object pattern - use a Null Object instance instead of null values. A lot of .NET Framework classes store the Null object as a static readonly field called Empty on the class itself. This makes for a bit cleaner code:

test.Email=test.Email??Email.Empty;

Another option is to initialize the Email property in the class's constructor. XSD generated classes are partial which allows you to define a constructor in a partial file that won't be overwritten when you regenerate the proxy, eg:

public partial class Test 
{
    public Test()
    {
        Email=new Email();
    }

}

Xsd.exe generated class doesn't serialize mandatory elements when null value

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