I just ran into a rather strange problem when I tried to rebuild my BizTalk solution. A bunch of these errors popped up:
Error 3 Source file 'C:\Source\MyProject\InputFile_XML.xsd.cs' could not be opened ('Unspecified error ')
Turns out the rebuild wipes out *.xsd.cs files. These files are generated by Visual Studio at build time for each schema file but when they are removed, subsequent rebuilds will fail to regenerate these .xsd.cs files and the compilation fails. This seems to be related to TFS because the solution is to check out all the schema files in the project and build again. The .xsd.cs files will be generated again.