I intend to cover some more foundational material for BizTalk
in the future, but today I wanted to cover an issue that at least some people
will run into fairly quickly when beginning to use the product.

There are times when it is desirable to work with multiple XML schemas that specify
the same target namespace, and which specify different definitions for the same element.

For instance, you may wish to have a lax schema when a document lands on your doorstep
initially – but further into the processing of that document (along a particular path)
you may wish to validate against a stricter schema.  Or, you may have a situation
where you have what is arguably an envelope structure which can’t be cleanly stripped
off (for a variety of reasons) – leaving you with documents which might look quite
different, but have the same target namespace and element usage.

BizTalk 2004, in general, wants to see one schema deployed to the BizTalk management
database for any given combination of target namespace and element declaration. 
If you deploy two schemas with target namespace http://MyNamespace and element declaration
MyRoot, and then attempt to receive a MyRoot-rooted document through a receive port
using the default Xml Receive pipeline, you will receive an error from BizTalk like
this one:

There was a failure executing the receive pipeline…Source:"XML Disassembler"…
Reason: The disassembler cannot retrieve the document specification using this type:
"http://MyNamespace#MyRoot". Either the schema is not deployed correctly,

or more than one schema is deployed for the same message type.

To overcome this, you can use custom BizTalk Pipelines on the send and receive ports
that will be dealing with schemas that are subject to the ambiguity.  Within
a pipeline, you can restrict the set of available schemas from “everything that is
deployed” down to the schema(s) that you are interested in.

Specifically, for receive ports, you can add a new item to your project (a “Receive
Pipeline”), and add the default Disassembler and Party Resolution pipeline components. 
For the Disassembler component, edit the “Document Schemas” collection and add the
particular schema you are interested in.  See this picture for
an illustration.

Likewise, for send ports, you can add a “Send Pipeline” item to your project, and
add the default Assembler pipeline component.  Again, specify the schema you
are interested in with the “Document Schemas” collection of the Assembler.

For each of the Send or Receive ports that will be trafficking in these messages,
specify your newly created pipelines – instead of the default Xml Receive/Send pipelines! 

Now for the gotchya! (you knew there had to be one, right?)

BizTalk 2004 will require that the assembly containing your custom pipeline(s) is
deployed to the GAC (along with every other BizTalk project assembly.)

When components loaded from the GAC wish to dynamically load other types & assemblies,
they must do so with fully qualified assembly names.  Applying this to our current
discussion means this: BizTalk pipelines must have fully qualified information for
the assembly that contains the schemas you configure within pipeline components.

If the pipeline component lives in the same BizTalk project as the schemas you are
attempting to reference, the property designer (when editing the “Document Schemas”
collection) will only be populated with a namespace-qualified type name – the fully
qualified assembly name will be missing.  At run time, the schemas will not be
found…and the behavior at run time will appear completely unchanged from the
case where no custom pipeline was specified at all. 

To work around this, simply put your pipelines in a different project/assembly
than the project containing the schemas you need to reference in the designer.

A bug you say?  Certainly it would be nice if the designer warned you,
and it deserves a KB article soon…But keep in mind what is happening: 
A GAC-destined component (a pipeline Disassembler) is providing designer support which
allows you to select another component (a schema) which will be loaded dynamically
at run time…It raises an interesing problem that goes beyond just BizTalk 2004.

Whenever a component that is destined for the GAC has IDE designer support which
in turn allows you to select a type for a “plug in” component that will be loaded
by the “host” component dynamically at run time
(without using Assembly.LoadFrom
semantics) – you will run into this issue.  Why?  Because if you select
a type from the same project, the fully qualified name can’t be reliably
known.  After all, the project might not have been compiled yet, or the fully-qualified
name might be set up to change with each compilation (gasp!) via 1.0.*.* versioning
policy.  If you use such a designer to select a type in a distinct assembly,
the fully qualified name can indeed be known -and shame on the component author if
the versioning policy isn’t sane.

Of course, being deployed into the GAC raises all kinds of thorny issues, but this
one was a bit subtle…