I received an external schema with the following type in it:

<xs:element minOccurs="1" maxOccurs="1" name="Id" type="wsdl:guid" />

There also was a reference to:

 <xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:wsdl="http://microsoft.com/wsdl/types/">
<xs:import namespace="http://microsoft.com/wsdl/types/" />

However the build failed with:
Error 6 Type ‘http://microsoft.com/wsdl/types/:guid’ is not declared, or is not a simple type. D:ExternalSchemasschemaname.xsd ExternalSchemas

I came ac cross a few posts for handling guids in schema’s:
http://www.level533.com/2011/01/how-to-define-guid-properties-in-xsd-schemas-for-generating-poco-classes/
http://msdn.microsoft.com/en-us/library/office/aa203890(v=office.11).aspx

The simplest way to solve this is to add a schema with the guid type in it and then add the location in the import. So add a schema:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:tns="http://microsoft.com/wsdl/types/" elementFormDefault="qualified" targetNamespace="http://microsoft.com/wsdl/types/" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:simpleType name="guid">
<xs:annotation>
<xs:documentation xml:lang="en">
The representation of a GUID, generally the id of an element.
</xs:documentation>
</xs:annotation>
<xs:restriction base="xs:string">
<xs:pattern value="[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}" />
</xs:restriction>
</xs:simpleType>
</xs:schema>

Then add the location to the import in the first schema:

<xs:import namespace="http://microsoft.com/wsdl/types/" schemaLocation="SimpleImport.xsd"/>

Builds!

Suddenly, I was getting these messages when opening an orchestration in BizTalk:

—————————
Microsoft Visual Studio
—————————
The ‘ODPackage’ package did not load correctly.

The problem may have been caused by a configuration change or by the installation of another extension. You can get more information by examining the file ‘C:UsersAdministratorAppDataRoamingMicrosoftVisualStudio11.0ActivityLog.xml’.

Continue to show this error message?
—————————
Yes No
—————————

There are a lot of solutions and suggestions in the web:
http://www.44342.com/biztalk-f20-t14861-p1.htm
http://geekswithblogs.net/synBoogaloo/archive/2007/04/17/111745.aspx
http://jglisson73.wordpress.com/2010/12/27/biztalk-orchestration-will-not-open-in-the-designer/

Of course all these didn’t work… A complete uninstall and install of BizTalk worked shortly. (If you do this, you can easily export and import the BizTalk settings in the configuration tool. Remember to delete the databases..) But then the problem reoccurred again.

In the solution there was an empty solution folder but I already removed it. The trick was: delete the complete solution, then do a get-latest on TFS. No problems again!! Solution worked!

It has to to with empty solution folders of files with no extension, in combination with TFS.