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.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.By default you will see a red border around the UI element when this happens, e.g.In a MVVM WPF application, the view model acts as the window’s Data Context: This means that that all controls inside the window will inherit its Data Context unless some parent element of a control overrides this by setting its own Data Context property.Besides inheriting and setting the Data Context property on an element directly you can also specify a binding source using the Element Name property, used when you want to bind to some other element, or the Relative Source property.
You can do this by implementing a custom validation rule and associate this with the object at any given step, any errors that were added to the Validation.
Note that the Text Block will appear on top of any elements that are located right below the Text Box as adorners are always visually on top.
Now that you can see the actual error message, which reads “Value …
A converter class converts data from one type to another during binding by implementing the Convert and Convert Back methods of the mentioned interface.
Once you have created a converter class you will typically add it as a resource in XAML with a unique x: Key attribute and then reference it from the binding as a Static Resource: (string) to a dependency property of type string you don’t necessarily need to use a converter as the default conversion will automatically apply the To String() method on the value of the source property.