ValidatingModelBase

Introduction

Imagine the scene... The user is filling out a form you've painstakingly written, and they enter their name where they should have entered their email address. You need to detect this, and display the problem in a clear way.

Input validation is a bit area, and there are many ways to go about it. The easiest and most appealing is to throw an exception in the setter for your property, like this:

private string _name;
public string Name
{
   get { return this._name; }
   set
   {
      if (someConditionIsFalse)
         throw new ValidationException("Message");
      this._name = value;
   }

When the binding sets this property, it notices if an exception is thrown, and updates the validation state of the control appropriately.

This, however, ends up being a thoroughly bad idea. It means that your property can only be validated when it's set (you can't go through and validate the whole form when the user clicks 'Submit', for example), and it leads to big fat property setters with lots of duplicated logic. Horrible.

C# also defines two interfaces, both of which WPF knows about: IDataErrorInfo and INotifyDataErrorInfo. Both of these provide a means for the ViewModel to tell the View, through events and PropertyChanged notifications, that one or more properties have one or more validation errors. Of these, INotifyDataErrorInfo is newer, easier to use, and allows for asynchronous validation.

However, driving INotifyDataErrorInfo is still a bit unintuitive: it allows you to broadcast the fact that one or more properties have errors, but provides no easy means for you to run your validations, and requires that you keep a record of what errors are associated with which properties.

ValidatingModelBase aims to solve this, and provide an intuitive and easy way of running and reporting your validations.

ValidatingModelBase

ValidatingModelBase derives from PropertyChangedBase, and is inherited by Screen. It builds on PropertyChangeBase's ability to notice when properties have changed to run and report your validations.

There are many ways to run validations, and many good libraries out there to help you. It isn't Stylet's intention to provide another validation library, so instead Stylet allows you to provide your own validation library to be used by ValidatingModelBase.

This manifests itself in ValidatingModelBase's validator property, which is an IValidatorAdapter. The intention is that you write your own implementation of IValidatorAdapter, which wraps your preferred validation library (I'll cover some examples of how to do this later), so that it can be used by ValidatingModelBase.