WPF Validating unbound textbox

Question

is it possible to use validation without the Binding part? The thing is my textbox is not bound to any object, but I still want to validate it's content. The only way I've found so far is this:

    <TextBox Grid.Row="0" Grid.Column="1" MaxLength="50" x:Name="textBoxTubeName" Margin="5,5,0,5">
        <TextBox.Text>
            <Binding Path="Name" UpdateSourceTrigger="PropertyChanged" Mode="TwoWay" NotifyOnValidationError="True">
                <Binding.ValidationRules>
                    <validation:InvalidCharactersRule />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>

But again, it only works when the TextBox.Text is bound to something (in this case, the Name property), how would I go about this without binding?

Thanks!

Answer 1

According to the MSDN forums it's not possible yet but it is planned (Note: this is an old post). However, I still can't find a way to do it so it may not be implemented yet.

Answer 2

It is a lot tricky to do from code-behind. Basically, you can set a temporary binding from code and raise the validation error and when the input has valid value you can remove all the temporary binding stuff again.

Here what I use, which I consider a bad practise (but it's better from nothing):

    /// <summary>
    /// Marks a textBox control as invalid (via validation error) from code.
    /// </summary>
    /// <param name="textBox">The text box.</param>
    /// <param name="errorContent">Content of the error.</param>
    public static void ValidationMarkInvalid(TextBox textBox, String errorContent)
    {
        DependencyProperty textProp = TextBox.TextProperty;
        if (!BindingOperations.IsDataBound(textBox, textProp))
        {
            if (textBox.DataContext == null)
            {
                textBox.DataContext = new EmptyDataContext();
            }

            Binding b = new Binding("CodeBehind");
            b.FallbackValue = textBox.Text;
            b.ValidatesOnExceptions = true;
            BindingOperations.SetBinding(textBox, textProp, b);
        }

        BindingExpression bindingInError =
            textBox.GetBindingExpression(TextBox.TextProperty);

        var validationError = new ValidationError(
            new EmptyValidationRule(),
            bindingInError,
            errorContent,
            new Exception(errorContent));

        Validation.MarkInvalid(bindingInError, validationError);
    }

    /// <summary>
    /// Clears the validation error from a textBox.
    /// </summary>
    /// <param name="textBox">The text box.</param>
    public static void ValidationClear(TextBox textBox)
    {
        DependencyProperty textProp = TextBox.TextProperty;
        if (BindingOperations.IsDataBound(textBox, textProp))
        {
            String value = textBox.Text;
            Validation.ClearInvalid(textBox.GetBindingExpression(TextBox.TextProperty));
            BindingOperations.ClearBinding(textBox, textProp);
            textBox.Text = value;

            EmptyDataContext ctx = textBox.DataContext as EmptyDataContext;
            if (ctx != null)
            {
                textBox.DataContext = null;
            }
        }
    }

    #region Nested Type: EmptyDataContext
    private sealed class EmptyDataContext : INotifyPropertyChanged
    {
        public Object CodeBehind
        {
            get
            {
                throw new FormatException();
            }
            set
            {
                throw new FormatException();
            }
        }

        #region INotifyPropertyChanged Members
        public event PropertyChangedEventHandler PropertyChanged;
        #endregion
    }
    #endregion

    #region Nested Type: EmptyValidationRule
    private sealed class EmptyValidationRule : ValidationRule
    {
        /// <summary>
        /// When overridden in a derived class, performs validation checks on a value.
        /// </summary>
        /// <param name="value">The value from the binding target to check.</param>
        /// <param name="cultureInfo">The culture to use in this rule.</param>
        /// <returns>
        /// A <see cref="T:System.Windows.Controls.ValidationResult"/> object.
        /// </returns>
        public override ValidationResult Validate(Object value, CultureInfo cultureInfo)
        {
            return new ValidationResult(false, String.Empty);
        }
    }
    #endregion

Now, from your code-behind you do this:

ValidationMarkInvalid(textBox, "Please enter something valid!");

and to clear the validation:

ValidationClear(textBox);

P.S.: If you don't want to validate on exceptions you can remove the EmptyDataContext class from the above methods.