The latter can for example be very useful for bindings in Styles and Control Templates and when you want to bind to some property in a parent element.
The Mode property of the , which is the default for the Text property of the Text Box control, the text you type into the Text Box does not update the source property until the control loses focus which happens when you click away from it. the setter for the bound property of the Data Context to get called, as the user is typing into the Text Box you set the Update Source Trigger property to enumeration.
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.
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.
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.
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.The adorned element in this case is the Text Box control itself and you include an in the control template where you want to leave space for it.The template above will cause the error message to be displayed in a Text Block below the Text Box.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.If the binding is setup correctly and the view model implements the interface to provide notifications when the data changes, the changes are automatically reflected in the elements in the view that are bound to it.Correspondingly, an underlying data value in the view model is automatically updated when the user modifies the bound value in the view.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.