0x80070057: "A structure that supports derivation may not be used in this context."

Question

I'm using a WWSAPI proxy to talk to a WCF service.   I had a basic interface working fine, but now I've made my interface a little more complex, I'm getting the above error at run-time when I invoke a method.  Tracing indicates the error is coming from WsCall, but gives no more details.  

I'm using inheritance which I presume is the source of the problem, even though the documentation claims that xsd extension is supported, and wsutil seems to handle the interface fine.   The code is a little complex to post - before I put the effort into producing a concise repro, does anyone know a likely cause of this error? 

Thanks

Andy

Answer 1

I've got this narrowed down a bit more.  The error occurs when I have a class that contains an array of base class objects, and I use the XmlElement attribute on the array, e.g.:

[Serializable]
class MyClass
{
     [XmlElement(ElementName = "BaseObject")]
     public Base[] BaseObjects;
}

The problem only occurs if Base has some derived classes, e.g.

[Serializable]
[XmlInclude(typeof(Derived))]
class Base
{
}

If I remove the XmlElement from the BaseObjects field, everything works fine.

At the XSD level, the problem form is:

<xs:complexType name="MyClass">
<xs:sequence>
  <xs:element minOccurs="0" maxOccurs="unbounded" name="BaseObject" type="tns:Base" />
</xs:sequence>
</xs:complexType>

whereas the working form is:

<xs:complexType name="MyClass">
  <xs:sequence>
    <xs:element minOccurs="0" maxOccurs="1" name="BaseObjects" type="tns:ArrayOfBase" />
  </xs:sequence>
</xs:complexType>

<xs:complexType name="ArrayOfBase">
  <xs:sequence>
    <xs:element minOccurs="0" maxOccurs="unbounded" name="Base" nillable="true" type="tns:Base" />
  </xs:sequence>
</xs:complexType>

It's not obvious to me why the latter works but the former doesn't.  But now that I know what causes the problem, I can work around it.

 

Answer 2

This is a known limitation in our current release. We are looking into supporting non-nillable array elements with base types in the next release.