MVVM number input validation

Question

I implemented the interface IDataErrorInfo (.NET 4) for validating user input. Thanks to this I'm able to verify minimum and maximum value. View Model therefore knows whether it is possible to continue with the next action (eg. start the calculation).

Question is, how to tell View Model, whether it is valid own entry of number? (User input cannot be parse to number.) In this case, public string this[string columnName] is not fired (of course...).

Is there any other possibility, except to drill down throw all controls like this?

private bool IsValid(DependencyObject obj)
{
	return !Validation.GetHasError(obj) && LogicalTreeHelper.GetChildren(obj).OfType<DependencyObject>().All(IsValid);
}

Best regards

Lukas

Answer 1

You won't be able to ever set an int property of the view model to a string value but you could implement a validation rule if you want to customize the error message that is displayed in the view when the conversion fails. Please refer to my blog post about data validation in WPF for more information about this: http://blog.magnusmontin.net/2013/08/26/data-validation-in-wpf/.

But again, you won't be able to set a view model property of type int to a string value. The setter will never be called. So you cannot handle this case in the view model.

Hope that helps.

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Answer 2

If you have errors in the view that stop it persisting to the viewmodel, say conversion errors, then you would need to tell the viewmodel about them.

You can see one way to do that in this:

http://social.technet.microsoft.com/wiki/contents/articles/28597.aspx

        <ControlTemplate x:Key="AddingTriggers" TargetType="ContentControl">
            <ControlTemplate.Resources>
                <Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource ErrorToolTip}">
                    <Setter Property="HorizontalAlignment" Value="Left"/>
                </Style>

            </ControlTemplate.Resources>
            <StackPanel>
                <i:Interaction.Triggers>
                    <local:RoutedEventTrigger RoutedEvent="{x:Static Validation.ErrorEvent}">
                        <e2c:EventToCommand   Command="{Binding ConversionErrorCommand, Mode=OneWay}"
                                              EventArgsConverter="{StaticResource BindingErrorEventArgsConverter}"
                                              PassEventArgsToCommand="True" />
                    </local:RoutedEventTrigger>
                </i:Interaction.Triggers>
                <TextBlock Text="This would be some sort of a common header" Foreground="LightBlue" HorizontalAlignment="Right"/>
                <ContentPresenter/> <!-- This allows you to put variable content "within" the control in XAML -->
                <TextBlock Text="This would some sort of a common footer" Foreground="LightBlue"  HorizontalAlignment="Right"/>
            </StackPanel>
        </ControlTemplate>
    </Application.Resources>

When you have an error raised or removed then an errorevent is raised.

That is passed to the viewmodel using EventToCommand.

That particular sample doesn't do much with it:

        // From Validation.Error routed Event
        private RelayCommand<PropertyError> conversionErrorCommand;
        public RelayCommand<PropertyError> ConversionErrorCommand
        {
            get
            {
                return conversionErrorCommand
                    ?? (conversionErrorCommand = new RelayCommand<PropertyError>
                        (PropertyError =>
                        {
                            if (PropertyError.Added)
                            {
                                TheResult=PropertyError.Error;
                            }
                            else
                            {
                                TheResult = "OK";
                            }
                        }));
            }
        }

This one does though:

https://gallery.technet.microsoft.com/WPF-Entity-Framework-MVVM-78cdc204

That's pretty complicated, but basically the errors are added to validation failures which are raised from dataannotations on each model and presented back to the view.

---

Hope that helps.

Technet articles: WPF: Layout Lab; All my Technet Articles

Answer 3

Thank you Andy,

that's exactly what I was looking for.