How to suppress validation when nothing is entered

Question

I use WPF data binding with entities that implement IDataErrorInfo interface. In general my code looks like this:

Business entity:

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Answer 1

I believe this behavior to also be a good solution. It removes ErrorTemplate on TextBox when needed and also supports multiple "valid" invalid values (you can also improve it by making ValidInputs a dependency property).

public class NotValidateWhenSpecified : Behavior<TextBox>
{
    private ControlTemplate _errorTemplate;

    public string[] ValidInputs { get; set; } = { string.Empty };

    protected override void OnAttached()
    {
        AssociatedObject.TextChanged += HideValiationIfNecessary;

        _errorTemplate = Validation.GetErrorTemplate(AssociatedObject);
        Validation.SetErrorTemplate(AssociatedObject, null);
    }        

    protected override void OnDetaching()
    {
        AssociatedObject.TextChanged -= HideValiationIfNecessary;
    }

    private void HideValiationIfNecessary(object sender, TextChangedEventArgs e)
    {
        if (ValidInputs.Contains(AssociatedObject.Text))
        {                
            if (_errorTemplate != null)
            {
                _errorTemplate = Validation.GetErrorTemplate(AssociatedObject);
                Validation.SetErrorTemplate(AssociatedObject, null);
            }                
        }
        else
        {
            if (Validation.GetErrorTemplate(AssociatedObject) != _errorTemplate)
            {
                Validation.SetErrorTemplate(AssociatedObject, _errorTemplate);
            }                
        }
    }
}

Answer 2

I've implemented the following solution:

 public class SkipValidationOnFirstLoadBehavior : Behavior<TextBox>
 {
        protected override void OnAttached()
        {
            AssociatedObject.LostFocus += AssociatedObjectOnLostFocus;
        }

        private void AssociatedObjectOnLostFocus(object sender, RoutedEventArgs routedEventArgs)
        {
            //Execute only once
            AssociatedObject.LostFocus -= AssociatedObjectOnLostFocus;

            //Get the current binding
            BindingExpression  expression = AssociatedObject.GetBindingExpression(TextBox.TextProperty);
            if (expression == null) return;
            Binding parentBinding = expression.ParentBinding;

            //Create a new one and trigger the validation 
            Binding updated = new Binding(parentBinding.Path.Path);
            updated.ValidatesOnDataErrors = true;
            updated.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;  
            AssociatedObject.SetBinding(TextBox.TextProperty, updated);
        }
 }

Example of usage:

    <TextBox Text="{Binding Email}">
        <i:Interaction.Behaviors>
            <local:SkipValidationOnFirstLoadBehavior/>
        </i:Interaction.Behaviors>
    </TextBox>