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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.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.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.
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.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 …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.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.