Filtered WPF textbox

Question

I am trying to make a textbox which filters user input to match specified type, so I can discard some of my validation logic.

For example, if I specify ushort I want my textbox to only accept text changes which result in a valid ushort value and nothing else.

This is what I've ended up with:

public abstract class CustomTextBox<T> : TextBox
{
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value", typeof(T), typeof(CustomTextBox<T>), new FrameworkPropertyMetadata(default(T), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.Inherits, OnValueChanged));
    public T Value
    {
        get { return (T)GetValue(ValueProperty); }
        set { SetValue(ValueProperty, value); }
    }

    protected override void OnPreviewTextInput(System.Windows.Input.TextCompositionEventArgs e)
    {
        if (_regex != null && !_regex.IsMatch(e.Text))
        {
            e.Handled = true;
        }
        else
        {
            base.OnPreviewTextInput(e);
        }
    }

    protected override void OnTextChanged(TextChangedEventArgs e)
    {
        if (String.IsNullOrEmpty(Text))
        {
            return;
        }

        T val;
        if (!TryParse(out val))
        {
            var index = CaretIndex;
            Text = _validText;
            CaretIndex = index > 0 ? index - 1 : 0;
            e.Handled = true;
        }
        else
        {
            Value = val;
            _validText = Text;

        }
    }

    protected CustomTextBox(string regexPattern = null)
    {
        if (!String.IsNullOrEmpty(regexPattern))
        {
            _regex = new Regex(regexPattern);
        }
        _validText = ToString(Value);

        Loaded += OnTextboxLoaded;
    }

    protected override void OnLostFocus(RoutedEventArgs e)
    {
        ValidateText();

        base.OnLostFocus(e);
    }

    protected virtual string ToString(T value)
    {
        return value.ToString();
    }

    protected abstract bool TryParse(out T val);

    private readonly Regex _regex;
    private string _validText;

    private void ValidateText()
    {
        T val;
        if (!TryParse(out val))
        {
            Text = ToString(Value);
        }
    }

    private void OnTextboxLoaded(object sender, RoutedEventArgs e)
    {
        ValidateText();
    }

    private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var tb = (CustomTextBox<T>)d;
        if (!e.OldValue.Equals(e.NewValue))
        {
            var str = tb.ToString((T)e.NewValue);
            if (!str.Equals(tb.Text))
            {
                tb.Text = str;
            }              
        }
    }
}

ushort implementation:

public sealed class UshortTextBox : CustomTextBox<ushort>
{
    public UshortTextBox()
        : base(@"^[0-9]$")
    {
    }

    protected override bool TryParse(out ushort val)
    {
        return UInt16.TryParse(Text, out val);
    }
}

I'm new to this whole custom-control-making, so if you see some rookie mistakes or ways to improve my code, please let me know.

Answer 1

I think you can avoid inheritance here ("Favor Composition Over Inheritance") and use Attached Behavior instead.

Also, did you take a look at MaskedTextBox from Extended WPF Toolkit (http://wpftoolkit.codeplex.com/wikipage?title=MaskedTextBox)?

Answer 2

I think what your code is missing the most is documentation and some better naming. The constructor takes a regex string (with a very generic name regexPattern) and you also have to override TryParse(). There is absolutely no indication of why both are needed, without looking at the source of CustomTextBox.

Also, the name CustomTextBox pretty much doesn't say anything, I think you should think of a better name for it, one that actually describes how is the type different from normal TextBox.