Base Classes are not passed to SchemaImporterExtension

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We are migrating an application from VS.NET 2003 to VS 2005. The client and
server have many types passed via web services that are shared in common
assemblies. In VS.NET 2003, we used a custom tool to parse the generated
Reference.cs files to point to the shared types.

For VS 2005, we have a SchemaImporterExtension. Some of the shared types are
derived from other shared types. We can filter out these derived classes, but
their parent classes are never handed to our extension, even though they are
in the WSDL as they have always been. So the parent classes are incorrectly
always re-defined in Reference.cs.

Our current workaround is a "hack" web method per web service mentioning all
needed base classes in its signature, which forces them to be passed to the
SIE. This is error-prone, since we have a large number of web services, which
use varying inherited types.

Note this is the opposite problem of child classes not being defined; no
amount of XmlInclude would likely fix this, since the WSDL is correct.

How can the parent classes be imported without resorting to dummy web methods?

Thanks!