While CM will enable Binding. ValidatesOnDataErrors if a view model implements IDataErrorInfo automatically, it does not provide any specific support for property validation. An example project is also available so you can see how it works. Added an example of validating a password entered using the PasswordBox element. The main property of the PasswordBox, Password, is not a dependency property so cannot be bound. This makes view model validation non-standard and not amenable to convention binding employed by Caliburn.
Added the concept of validation groups.
It might be used to independently validate the properties on each page of a TabControl. Added support for validating only when a guard property is enabled. By default the guard property is Can[PropertyName].
But the guard property a validation controller will use can be set explicitly. The application has been created using VS and. To compile you will need to add a reference to Caliburn. This example shows how property validation can be defined for any CM screen just by decorating the properties with ValidationAttribute instances such as [Required ], [Range] or your own custom validation attributes. However I think its fairly common, especially in quick and dirty application written to get a job done, that either:.
This example sub-classes the CM Screen implementation to provide a class from which your view model can be derived.
This mechanism makes it really simple to validate property values and provide a corresponding validation failure message so WPF error templates will be activated automatically. All you have to do is have your view model inherit from ValidatingScreen and decorate properties which are bound to your view. You might create a view model class like irrelevant details omitted:. Even the bulk of this example is boilerplate code. The relevant lines are the validation attributes decorating the EmailAddress property and EmailValidator.
With these declarations the ValidatingScreen implementation will ensure errors are raised when either condition is not valid. It will also ensure the corresponding message is returned by the IDataErrorInfo.
Item property so WPF can report it. Part of this simplicity is possible because of the validators used. The validators are instances of sub-classes of the abstract ValidationAttribute, a class introduced in.
A ValidationAttribute instance performs much the same role as ValidationRule but can be declared on the affected property and so, importantly, are part of the view model. This means all unit testing can include the effect of validation errors. A custom ValidationAttribute is easy to define and can be as complex as required.
The EmailValidatorAttribute used in the example shows how simple it can be to roll your own validator. This example is lifted from Scott Guthries blog post which introduces this decorative style of property validation. Importantly for me, a European,
Validating user input wpf mvvm william base ValidationAttribute class supports localization.
The exmaple above uses hard coded error messages. But when a ValidationAttribute instance is used to decorate a property the string to report can be defined in another class, for example a string resource, by using the option to specify a class and property from which the string should be retrieved.
The view model of example application attached to this post shows this option being used. The ValidationAttribute class can also be used to decorate methods and method parameters but these two features are not by the ValidatingScreen implementation.
You can still declare validators this way but they will be ignored by the ValidatingScreen class. Generating an Validating user input wpf mvvm william and message is one part of the example.
The other is reporting it in the view. The example uses the control error template created by Beth Massi in her post about displaying data validation messages in WPF. In her article Beth creates a reusable template defined to be used as a static resource in Xaml which can then be applied as the Validation.
ErrorTemplate for many controls. When referenced for the first time when your view model is created static properties of the ValidatingScreen class create two collections: Here is are the static variable assignments:. The indexer makes use of the property and validator lists created when the ValidatingScreen class is first initialized to perform the checks. It calls the IsValid method of each validator attribute which decorates the property being tested and passes to the IsValid method the current value of the property.
All validators for a property will be tested and a collection of any error messages is returned from the indexer as one string with each error message on its own line. The class also implements the Error property of the IDataErrorInfo interface which is used to test all validators of all properties. This can be used to set the state of a property which is bound to the IsEnabled property of, say, a button control.
The example application shows how this might implemented and is fairly trivial:.
And this is how the example is implemented except the view model property is called NoValidationErrors not IsValidated. The main idea for this example comes from this post on the Caliburn page on Codplex. Download the example project. How can you stop firing these validations on the screen load. One of the challenges of WPF I find is that the error reporting mechanism is buried. It relies on the values of attached properties defined in the System. The properties attached to controls are set by the notification mechanism.
So I suspect you need to suppress calls to NotifyOfPropertyChange until you really want errors to appear. May be use a flag or something to disable calls "Validating user input wpf mvvm william" the screen is Validating user input wpf mvvm william. What about validation in Silverlight?
The bool IsValid method is not existing in SL. Could your code be used in SL too? Whar has to be changed? In principle, yes, it should be possible to use it in Silverlight. This relies on overriding the CM Shell and checking a properties value against the set of ValidationAttribute instances decorating a property. If so it would not surprise me. The idiotic and needless changes to WPF classes as they appear in Silverlight is one of the reasons I stopped using Silverlight.
Can you look into this please? I found your article from the Caliburn. Micro codeplex discussion forums. Are you still using the ValidatingScreen? If so, have you made any updates in the past year? Apart from a few changes to accommodate error reporting, there are no differences. We do use the screen but… For the past year and for the foreseeable future there have been no plans to do further WPF work.