Text on TextBox with UpdateSourceTrigger=PropertyChanged is not updated when coercion of text input results in unchanged source value

Question

I have a text box whose Text property has a TwoWay MultiBinding with UpdateSourceTrigger set to PropertyChanged. The first Binding is to a dependency property (Value) which has a PropertyChangedCallBack function that rounds the value to one decimal place.

The purpose of the text box is to perform the rounding as the user types rather than when the text box loses focus, hence why UpdateSourceTrigger is set to PropertyChanged.

The problem I am having is that if text is entered that does NOT result in Value changing, the Text property and Value become out of sync. Only if the rounding operation causes Value to change does Text get updated on the fly. E.g., if Text and Value are both 123.4 and the user types 1 after this, Value is rounded to the same value (123.4), but Text shows 123.41. However, if 9 is then typed after the 4, Value is rounded up to 123.5. And because of this actual change, Text is then updated to the same (123.5).

Is there any way of forcing a text box to update from its source even when the source hasn't changed since the last trigger? I have tried using BindingExpressionBase.UpdateTarget() but this only works when UpdateSourceTrigger is set to Explicit, which can't be used as Value no longer gets updated prior to a suitable time where UpdateTarget could be called (such as a TextInput handler). I have tried other methods such as explicitly updating the Text value from the bound Value, forcing an actual change to Value temporarily to invoke an update, but these "hacks" either don't work or cause other problems.

Any help would be greatly appreciated.

The code is below.

XAML snippet

<TextBox>
  <TextBox.Text>
    <MultiBinding Converter="{local:NumberFormatConverter}"
                  UpdateSourceTrigger="Explicit"
                  Mode="TwoWay">
      <Binding Path="Value"
               RelativeSource="{RelativeSource AncestorType={x:Type Window}}"
               Mode="TwoWay" />
    </MultiBinding>
  </TextBox.Text>
</TextBox>

C# snippet

public static readonly DependencyProperty ValueProperty =
    DependencyProperty.Register(
        "Value", typeof(decimal), typeof(MainWindow),
        new FrameworkPropertyMetadata(0m,
        new PropertyChangedCallback(OnValueChanged)));

private static void OnValueChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
    obj.SetValue(ValueProperty, Math.Round((decimal)args.NewValue, 1));
}

Converter class required

public class NumberFormatConverter : MarkupExtension, IMultiValueConverter
{
    public static NumberFormatConverter Instance { private set; get; }

    static NumberFormatConverter()
    {
        Instance = new NumberFormatConverter();
    }

    public override object ProvideValue(IServiceProvider serviceProvider_)
    {
        return Instance;
    }

    #region Implementation of IMultiValueConverter

    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        return values[0].ToString();
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        var result = 0m;
        if (value != null)
        {
            decimal.TryParse(value.ToString(), out result);
        }
        return new object[] { result };
    }

    #endregion
}

Answer 1

I did a little digging on the Internet, and it turns out this was broken in WPF 4. Someone with an almost-identical problem to me posted here: http://www.go4answers.com/Example/textbox-shows-old-value-being-coerced-137799.aspx

'Answer 8' states this was broken in WPF 4 and suggests a solution, which is to actually use UpdateSourceTrigger="Explicit" but to handle the TextChanged event and call BindingExpression.UpdateSource() to force changes in the text box to be reflected in the underlying value as if UpdateSourceTrigger="PropertyChanged", as per this post: Coerce a WPF TextBox not working anymore in .NET 4.0

I implemented this, but lo and behold there were further side effects, in particular that every keystroke caused the caret to jump to the start of the text box due to updating the source and raising a PropertyChanged event. And also, any leading or trailing zeros or decimal places entered with the intention of entering further digits would get wiped out immediately. So, a simple condition to check the parsed decimal value of the text box versus the underlying value resolved this.

The following event handler is all that was needed:

private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
{
    var tb = (TextBox)e.Source;
    MultiBindingExpression binding = BindingOperations.GetMultiBindingExpression(tb, TextBox.TextProperty);

    decimal result = 0m;
    decimal.TryParse(tb.Text, out result);

    if ((decimal)GetValue(ValueProperty) != result && binding != null)
    {
        int caretIndex = tb.CaretIndex;
        binding.UpdateSource();
        tb.CaretIndex = caretIndex;
    }
}