Silverlight Custom Validation

I have a Silverlight App in which I am using data binding, INotifyPropertyChanged and CustomValidation to perform validation on my input screens.

On one of my input screens the custom开发者_Python百科 validation is dependent on a category the user selects. Based on the category, certain fields become mandatory or not.

The way my input screens work, incorrect input fields get a red box around them when they are not valid. My problem however, is that the validation in the background is working but it does not update on the screen, i.e. the user has no visual que that a certain field is now mandatory.

Fields which become mandatory as a result of a category change, do not get the red box around them unless the user goes into the field and change it.