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.
if you are typing a letter into a Text Box bound to a source property of type int: The actual message that is describing the error is stored in the Error Content property of a objects in it, another attached property named Validation. To be able to see the error messages in the view you can replace the default control template that draws the red border around the element with your own custom template by setting the Validation.
Error Template attached property of the control: Note that the Validation.
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.
Provided that the view model has a property called “Name”, you bind it to a Text Box’s Text property in XAML the following way: Besides the path that specifies the name of the property to bind to, the binding must also have a source object.
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.
Similarly, it makes it possible to invalidate a property when setting another property and it also supports setting multiple errors per property and custom error objects of some other type than that contains validation errors for the specified property or for the entire entity.
You should always raise the Errors Changed event whenever the collection returned by the Get Errors method changes.
This interface defines two properties that returns a string indicating what is wrong with the object and some property of the object respectively.
Below is how the view model would implement the interface – the same interface has been present in Silverlight since version 4 – which enables you to perform server-side validations asynchronously and then notify the view by raising an Errors Changed event once the validations are completed.