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If the source of a two-way binding implements the interface and the Validates On Notify Data Errors property of the binding is set to true (which it is by default), the WPF 4.5 binding engine automatically monitors the Errors Changed event and calls the Get Errors method to retrieve the updated errors once the event is raised from the source object provided that the Has Errors property returns true.
Below is an example of a simple service with a single method that validates a username by first querying a database to determine whether it is already in use or not and then checks the length of it and finally determines whether it contains any illegal characters by using a regular expression.
The setter of a Username property of the view model is then using a private method to call the service method asynchronously using the async and await keywords – these were added to introduce a simplified approach to asynchronous programming in the .
NET Framework 4.5 and the Windows Runtime (Win RT) – and update the dictionary based on the result of the validation: For the view to be able show more than a single error message you have to make some changes to the Validation. You typically use an Items Control present a collection of items in XAML: As mentioned, you can also return error objects of a any type from the Get Errors method and this can be very useful when you want to present custom error reporting in the view.
This means that you don’t have to use a converter to display a If a user enters an invalid value that cannot be converted to an int and be set as the value for the Age property in the view model in the above example, a validation error will occur and a visual feedback will be provided to the user to indicate this.
You should always raise the Errors Changed event whenever the collection returned by the Get Errors method changes.Error Template will be displayed on the adorner layer.Elements in the adorner layer are rendered on top of the rest of the visual elements and they will not be considered when the layout system is measuring and arranging the controls on the adorned element layer.Errors attached property of the bound element during that step in a previous validation procedure are removed. Errors collection is also cleared when a valid value transfer occurs.WPF ships with two built-in concrete implementations of the object to the Validation.
A common requirement for any user interface application that accepts user input is to validate the entered information to ensure that it has the expected format and type for the back-end to be able to accept and persist it.