If the schema imports only the namespace of other schema without specifying the schema location and a catalog is set-up that maps the namespace to a certain location both the validation and the diagram will correctly identify the imported schema.
If the validation action finds that the schema contains unresolved references, the application will suggest the use of validation scenarios, but only if the current edited schema is an XML Schema module.
Also, a commenter below points to this more complete wrapper utility.
This proves to be very useful in avoiding setting invalid XML names for the given property. To resolve an error, just click it (or double-click for errors located in other schemas) and the corresponding schema component will be displayed as the diagram root so that you can easily correct the error.
Our development language is C if that makes any difference, although I don't think it should as we could generate the xml file and then do validation by calling a separate program in the test.
After some research, I think the best answer is Xerces, as it implements all of XSD, is cross-platform and widely used.
*The maximum size limit for file upload is 2 megabytes.
In this page you can check the validity of your XML Schema (XSD) file. The library used in this page is jaxp, Java API for XML Processing, version 1.4, from Java(TM) SE Runtime Environment (build 1.7.0_06-b24).