Using DataAnnotations for validation in MVVM

Question

Hi,

I've discovered the new data annotation features of SL3 and I'm using them for user input validation.

I've got inputs like these:

            <dataInput:Label  Target="{Binding ElementName=inputName}"/>
            <TextBox 
                x:Name="inputName" 
                Text="{Binding RequestDemoData.Name, Mode=TwoWay, ValidatesOnExceptions=true, NotifyOnValidationError=true}"/>
            <dataInput:DescriptionViewer {Binding ElementName=inputName}"/>

and my model looks like that:

   [Display(ResourceType = typeof(Resources.Resources), Name = "Name", Description = "NameDescription")]
    [Required(ErrorMessageResourceType = typeof(Resources.Resources), ErrorMessageResourceName = "NameRequired")]
    [RegularExpression(@"^[^0-9]*[a-zA-Z]+[^0-9]*$", ErrorMessageResourceType = typeof(Resources.Resources), ErrorMessageResourceName = "NameError")]
    public string Name
    {
        get
        {
            ValidateProperty("Name", _name);
            return _name;
        }
        set
        {
            if (_name != value)
            {
                ValidateProperty("Name", value);
                _name = value;
                OnPropertyChanged("Name");
            }
        }
    }

So far, so good. If the user inputs some wrong data, I get an error message when he/she focuses out. The issue is that I've got a submit button bound to an ICommand and I can't work out how to make the error message appear when the user clicks it.

The bad way is to add some code-behind and do GetBindingExpression(foo).UpdateSource() and that would sort it out. The downside is that it's completely unmanageable and I hate to habe code-behind on my viewa.

http://www.thejoyofcode.com/Silverlight_Validation_and_MVVM_Part_II.aspx proposed a solution and I'm going to follow it but I'd like to know if there isn't an easier way.

Cheers.

Answer 1

Unfortunately, there is not much of a better way to do this. The only way to have the UI update itself based on validators is in the setter of the binding.

This, I believe, is a huge limitation of the validation system in Silverlight. That JoyOfCode article is really the best way to go about it.

I would also recommend the article by the same publisher where you can bind errors to your viewmodel, but it doesn't work the other way around.

Answer 2

I have also used Josh's approach on a very large scale LOB application and whilst it is messy it does work. The Validation Context in particular is likely to get you out of a few scraps with more complex logic.