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One of the projects that I’m working on involves putting together a web service interface to a booking system so that business partners can integrate our inventory into their business systems. This involves exposing a fairly flexible schema so that different partners can supply different information. This means that for some requests some information is optional while for requests from other partners that information might be required.

Using ASP.NET web services, we implement this by having a class to represent the whole request which contains object references to other classes representing the optional information. In the schema, we get a minOccurs=”0” and maxOccurs=”1” and this works well.

One of the things that I learnt this week, however, that hadn’t been immediately apparent is that there is no way to insist that a sub-object must be present: an [XmlRequired] attribute if you will. This means that all reference types are optional.

According to Douglas Purdy, this behaviour is by design:

_If you already know what you want on the wire, why don't you define the WSDL first? You can use the wsdl.exe tool with the /server option to generate the code._

He says this Usenet post will be taken as a feature request - I presume nothing has been done yet and that this therefore may or may not exist in v2.0. It sure would be useful. Keeping our WSDL in sync with our code manually would be a pain, but we’d like the WSDL schema to represent most of the contract to our customers.