XML: Nillable="True" vs MinOccurs="0"

This came up at work today, as a coworker was strugling with a WSDL for an outside vendor. I only knew it because BizTalk runs into exactly this kind of error occassionally. The situation is this: A WSDL is defined with several elements. Let's use this for an example:

What you see here is a bunch of elements in an which my coworker is trying to show as null by not including. That is, when, for instance, "address" is null, it simply will not exist in the XML document. Here's how the same code looks as our vendor expects it.

Now, leaving aside the question of why the only way to pass data to the web method is allowed to be null, we can see that the vendor's wsdl expects the nodes to exist. Therefore, instead of simply leaving out "address" the XML document should contain a node like this: <> It makes sense when you think about it in terms of code as well. How many times in C# do we pass something like "foo = new foo(new bar(), null)?" Is there any question that this is different from "foo = new foo(new bar())"? Not really. (At least, not until C# 2zillion when Microsoft decides that any parameter which can accept a NULL is automatically an optional parameter.) I did manage to help steer the coworker in the right direction, and provided him (as I am you) a pretty good synopsis on this whole thing from IBM's website. Link here. (http://www.ibm.com/developerworks/xml/library/ws-tip-null.html) So, in summary, pay attention to your WSDL and Schemas. Nillable is not the same as MinOccurs="0".