How to get invalid values in a WPF view

Question

If I have a WPF view with a textbox that has a binding to a decimal (or any other number format) I automatically get a visual hint if I enter a letter or any other invald character and the value is not transferred to the viewmodel (the breakpoint on the setter is never reached). If I enter a number, everything works fine. To disable my save-Button (ICommand), I would like to get the info in my viewmodel that there is an error in the view in a MVVM-like fashion. Hints to where this behavior is documented are very welcome!

So the target situation looks like this:

what I want would be a disable "save and close":

XAML:

    <TextBox Text="{Binding Path=SelectedItem.Punkte_Seite_max, UpdateSourceTrigger=PropertyChanged}"/>

ViewModel

  public int Punkte_Seite_max
{
  get { return _punkte_Seite_max; }
  set
  {
    _punkte_Seite_max = value;
    Changed(); //INotifyPropertyChanged  call
  }
}

Answer 1

What you want to be using is INotifyDataErrorInfo documentation found here. This lets you provide custom validation on the properties that you have bound to your ViewModel.

This is a sample I have shamelessly copied from CodeProject but I have done so to prevent any link rot. I have also tried to adapt it slightly to match your example.

ViewModel

public class ViewModel : INotifyDataErrorInfo
{
    // A place to store all error messages for all properties.
    private IDictionary<string, List<string>> propertyErrors = new Dictionary<string, List<string>>();

    public string Preis
    {
        get { return _preis; }
        set
        {
            // Only update if the value has actually changed.
            if (!string.Equals(_preis, value, StringComparison.Ordinal))
            {
                _preis = value;
                Changed();

                this.Validate();
            }
        }
    }

    // The event to raise when the error state changes.
    public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;     

    // A method of getting all errors for the given known property.
    public System.Collections.IEnumerable GetErrors(string propertyName)
    {
        if (propertyName != null)
        {
            if (propertyErrors.TryGetValue(propertyName, out var errors))
            {
                return errors;
            }
        }

        return null;
    }

    // Whether there are any errors on the ViewModel
    public bool HasErrors
    {
        get
        {    
            return propertyErrors.Values.Any(r =>r.Any());
        }
    }

    private void Validate()
    {
        // 1. HERE YOU CAN CHECK WHETHER Preis IS VALID AND ANY OTHER PROPERTIES
        // 2. Update the 'propertyErrors' dictionary with the errors
        // 3. Raise the ErrorsChanged event.
    }
}

XAML

You will need to change your XAML to something like this:

<TextBox>
    <Binding Path="Preis" UpdateSourceTrigger="PropertyChanged" ValidatesOnNotifyDataErrors="True"/>
</TextBox>

Answer 2

Thanks to Bijington I got on the right track and found an answer which satisfies MVVM and also doesn't need code behind. In case someone is interested here's my solution to this issue.

The error shown above is created in the view because there is no converter in WPF from letters to int (how should there be one). To raise this issue the binding in needs to have NotifyOnValidationError=True.

 <TextBox Text="{Binding Path=SelectedItem.Punkte_Seite_max, UpdateSourceTrigger=PropertyChanged, NotifyOnValidationError=True}"

This raises a bubbling up Validation.Error event that can be captured anywhere in the tree. I decided to capture it via a routed event trigger like so: XAML:

<Window
...
xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity" >
    <i:Interaction.Triggers>
    <userInterface:RoutedEventTrigger RoutedEvent="{x:Static Validation.ErrorEvent}" >
        <userInterface:ViewErrorCounterAction ViewErrorCounter="{Binding Path=ViewValidationErrorCount, Mode=TwoWay}"/>
    </userInterface:RoutedEventTrigger>
</i:Interaction.Triggers>

So the twoway-binding is the MVVM-okayish link to my viewmodel.

ViewErrorCounterAction is based on this SO answer:

public class ViewErrorCounterAction : TriggerAction<DependencyObject> {
public ViewErrorCounterAction()
{
    ViewErrorCounter = 0;  // initalize with 0 as there should not be such errors when the window is loaded
}

public int ViewErrorCounter
{
    get
    {
        return System.Convert.ToInt32(GetValue(ViewErrorCounterProperty));
    }
    set
    {
        SetValue(ViewErrorCounterProperty, value);
    }
}

public static readonly DependencyProperty ViewErrorCounterProperty = DependencyProperty.Register("ViewErrorCounter", typeof(int), typeof(ViewErrorCounterAction), new PropertyMetadata(null));

protected override void Invoke(object parameter)
{
    var e = (ValidationErrorEventArgs)parameter;
    if ((e.Action == ValidationErrorEventAction.Added))
        ViewErrorCounter = ViewErrorCounter + 1;
    else if ((e.Action == ValidationErrorEventAction.Removed))
        ViewErrorCounter = ViewErrorCounter - 1;
}
}

Finally routed Event Trigger is based on https://sergecalderara.wordpress.com/2012/08/23/how-to-attached-an-mvvm-eventtocommand-to-an-attached-event/

Hope this helps and I'd appreciate comments on how to better solve this issue if there are more elegant ways :)