In my last blog I mentioned issues with BizTalk 09 when creating references between projects. Here's some follow up thanks to Mandi on the BizTalk escalation team.
BizTalk solutions are often separated into various projects for the different artifacts, pipelines, schemas, orchestration, etc. When working with a BizTalk project with references to other BizTalk projects compilation fails.
A review the project shows an orchestration variable is missing and shows a red exclamation mark. When selecting the type, it shows the referenced BizTalk project but no defined types. This is a known issue and a fix is available (no link available at this time).
Maps are not immune. If there is a map using a schema from a referenced BizTalk project, the "Schema Type Picker" in the map doesn't show the referenced project and schemas. It only shows local schemas.
To resolve this problem check to see if the "Build Action" is set to "None" instead of "BtsCompile" for the schema. Here's the link with more information.
Issues with BizTalk 2009 on VS.NET 2008
There may be other related symptoms. The problems mentioned here are not from a single case so check with the BizTalk team regarding referencing issues and BizTalk 09.