Interoperability Gotcha: Order of XML Elements

@YaronNaveh

This time I will talk about the importance of order within xml elements.

Let's say we have a web service with this schema:

<s:element name="root">
 <s:complexType>
  <s:sequence>
   <s:element name="elem1" type="s:string" />
   <s:element name="elem2" type="s:string" />
  </s:sequence>
 </s:complexType>
</s:element">

So of course this is a valid request:

<root>
 <elem1>I'm elem1</elem1>
 <elem2>I'm elem2</elem2>
</root>

But about about this?

<root>
 <elem2>I'm elem2</elem2>
 <elem1>I'm elem1</elem1>
</root>

This is not a valid xml instance!
The reason is that the schema contains the element which requires its sub elements to have order.

So why is it an interoperability gotcha?
The reason is that different soap stacks by different vendors behave differently in such cases. Sometimes even different stacks of the same vendor are divided...

Let's see how a .Net 2.0 proxy would look like:

...
private string elem1;
private string elem2;
...

This proxy can parse both xml instances from above.

This is how the equivalent WCF proxy would look like:

[XmlElement(Order=0)]
public string Elem1
{
...
}

[XmlElement(Order=1)]
public string Elem2
{
...
}

This proxy is stricter and actually enforces the order of elements. So it would only accept the first (legal) instance and will give unexpected results with the second. It would usually not throw an exception but rather ignore the message and use default values for the unordered elements so your fields will have a lot of NULL's and zeros.

How to fix it?

Option 1 (recommended) - Send XML with the correct order.

Option 2 - Change the WSDL to use <all> instead of <sequence>. The former does not require correct order of elements (there are some other subtle differences). Note that in case the WSDL is dynamically generated this may be hard to maintain.

Option 3 - Remove the "Order=..." property from the WCF proxy. Note that when the proxy will be regenerated the "Order" property will come back so this is also a maintenance challenge.

@YaronNaveh

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