A simple helper to manually toggle WPF validation on FrameworkElements.

ManualValidation.cs

/// <summary>
/// Provides helpers to manually toggle validation errors.
/// </summary>
public static class ManualValidation
{
    // this dummy attached property is used as a source
    // of the binding.
    static readonly DependencyProperty DummyProperty =
            DependencyProperty.RegisterAttached("DummyProperty", typeof(object), typeof(ManualValidation), new PropertyMetadata(null));

    // this class implements a dummy validation rule without behaviour.
    private class DummyValidationRule : ValidationRule
    {
        public override ValidationResult Validate(object value, CultureInfo cultureInfo)
        {
            throw new NotImplementedException();
        }
    }

    /// <summary>
    /// Manually mark a validation error on the given framework element.
    /// </summary>
    /// <param name="element">The instance of <see cref="FrameworkElement"/> on which to mark the validation error.</param>
    /// <param name="errorContent">An object representing the content of the error.</param>
    public static void MarkInvalid(FrameworkElement element, object errorContent)
    {
        // create a dummy binding. Conveniently, we bind to the tag of the FrameworkElement,
        // so as to minimise the potential interaction with other code.
        var binding = new Binding("Tag") {Source = element, Mode = BindingMode.OneWayToSource};

        // set the binding on to our dummy property.
        BindingOperations.SetBinding(element, DummyProperty, binding);

        // we now get the live binding expression.
        var bindingExpression = element.GetBindingExpression(DummyProperty);

        // create a dummy binding error, with the specified error content.
        var validationError = new ValidationError(new DummyValidationRule(), binding, errorContent, null);

        // and manually set the validation error on the binding.
        Validation.MarkInvalid(bindingExpression, validationError);
    }

    /// <summary>
    /// Clears all manually assigned errors on the given <paramref name="element"/>.
    /// </summary>
    /// <param name="element">The instance of <see cref="FrameworkElement"/> on which to clear the validation.</param>
    public static void ClearValidation(FrameworkElement element)
    {
        // to clear an error, we simply remove all bindings to our dummy property.
        BindingOperations.ClearBinding(element, DummyProperty); 
    }

}

It worked like a charm! Thank you very much!!

Very useful, thank you !

Useful even after 9 years. Thank you very much!