In his announcement of the March release of DSL Tools, Gareth mentioned that we now have a designer definition (DD) file validator. This validates the DD file for anything that is not caught by XSD validation, including whether the cross references to the domain model are correct. It also validates those aspects of the domain model which impact the mapping of the designer definition to the domain model. For example, it will check that the XML Root class is mapped to the diagram defined in the DD file. Errors and warnings appear in the Visual Studio errors window whenever you try to generate code from the DD file (i.e. any of the code generators in the Designer project) and disappear the next time your try if the error has been fixed.
You may not have realized this, but the domain model designer also includes some validation. It gets invoked whenever you try to save the file, or you can invoke it from the ValidateAll context menu. Try, for example, giving two classes the same name and then invoking ValidateAll.
As Gareth indicated, this validation is implemented on top of a validation framework, that we will be leveraging to allow users to include validation as part of their own designers, or calling directly through the API, from within a text generation template, for example, as a precondition to code generation. We'd be interested to hear from users about what they would do with such features, whether they think this to be an important set of features (customers we so far have talked with do), what kind of authoring experience they would expect or want, and any suggestions for other features in this general area (for example, how else would you like validation to be exposed through the UI of a designer). You can provide this feedback as comments to this posting, as comments to Gareth's post, or through the DSL Tools newsgroup, or as suggestions through the feedback center.