Combining ValidationRule and Value Converters for a textbox

Question

I have a simple problem that I just cant find a good solution for. I have a textbox bound to a double property value. The user can enter values into the textbox, but I only want to allow values between 0 and 100. I would like to show a red box around the textbox if an invalid value is entered while the textbox still has focus (UpdateSourceTrigger="PropertyChanged"). Should the user click away from the textbox, I want to clamp the value using a value converter on UpdateSourceTrigger="LostFocus".

Its easy to do either the validation rule or the converter, but I can not combine them as I want the validation to trigger on UpdateSourceTrigger="PropertyChanged" and the converter should trigger on UpdateSourceTrigger="LostFocus". Unfortunately I can only choose either one or the other when setting up the binding on my TextBox.Text.

Any good ideas about how I could implement this functionality?

Thank you

/Peter

Answer 1

That's an interesting question. I'm not sure I have a complete solution, but I'd like to throw out a couple ideas.

What do you think of creating a new class that derives from TextBox? It could have two dependency properties, MinValue and MaxValue. Then it could override OnLostFocus. (Disclaimer: I haven't tested the following code.)

public class NumericTextBox : TextBox
{
    public static readonly DependencyProperty MinValueProperty =
        DependencyProperty.Register("MinValue", typeof(double), typeof(NumericTextBox), new UIPropertyMetadata(Double.MinValue));

    public static readonly DependencyProperty MaxValueProperty =
        DependencyProperty.Register("MaxValue", typeof(double), typeof(NumericTextBox), new UIPropertyMetadata(Double.MaxValue));

    public double MinValue
    {
        get { return (double)GetValue(MinValueProperty); }
        set { SetValue(MinValueProperty, value); }
    }

    public double MaxValue
    {
        get { return (double)GetValue(MaxValueProperty); }
        set { SetValue(MaxValueProperty, value); }
    }

    protected override void OnLostFocus(System.Windows.RoutedEventArgs e)
    {
        base.OnLostFocus(e);

        double value = 0;

        // Parse text.
        if (Double.TryParse(this.Text, out value))
        {
            // Make sure the value is within the acceptable range.
            value = Math.Max(value, this.MinValue);
            value = Math.Min(value, this.MaxValue);
        }

        // Set the text.
        this.Text = value.ToString();
    }
}

This would eliminate the need for a converter, and your binding can use UpdateSourceTrigger=PropertyChanged to support your validation rule.

My suggestion admittedly has its drawbacks.

1. This approach would require you to have validation-related code in two places, and they'd need to match. (Maybe you could override OnTextChanged too, and set the red border there instead.)
2. This approach requires you to put rules in the view layer rather than in business objects, which you may or may not find acceptable.