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The method returns true or false depending on whether the validation succeeded or not and it also returns a collection of error messages as an out parameter.Declaring an argument as out is useful when you want the method to return multiple values.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.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.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 …

This post is about how data validation works in WPF and the different validation options there are available including implementing custom Validation Rules and using the In a typical WPF application that uses the MVVM (Model-View-View Model) design pattern, a dependency property of a user interface control in a XAML-defined view uses data binding to bind to some data returned by a CLR property of the view model.

Setting the property to this value means that the value of the source property only gets updated when you explicitly call the Binding Expression. You will typically never use this approach in an MVVM application though.

If you want to bind a view model property of a specific type to a dependency property in the view of a different type you may need to implement a custom converter class by implementing the interface and setting the Converter property of the binding to an instance of this.

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.

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