Sometimes it is useful to add our own validation messages to a Silverlight control, normally because we are doing custom validation through code, not through the reguar IDataErrorInfo, INotifyDataErrorInfo or ValidationAttributes. This is not straightforward, but it is possible. An example might be:
In this example I am using the TagProperty, because it is seldom used, but you can use whatever you like. The trick here is to force the update of a bound object, which throws an exception, which in turn is propagated as a validation error.