Can I change the properties of a binding in a DataTrigger, without knowing the binding itself?

Question

I have a TextBox style that formats a number if the box is unfocused, but leaves the number unformatted whlie it's being edited.

This is the style I want for multiple number TextBoxes, but they all contain different Text bindings. The only difference between the regular Text setter and the Triggered Text setter is that the Triggered one has StringFormat=N2 in the binding.

Is there a way I can make this style generic, such as only changing the StringFormat property of the binding in the DataTrigger?

<TextBox>
    <TextBox.Style>
        <Style TargetType="{x:Type TextBox}">
                <Setter Property="Text" Value="{Binding SomeValue, StringFormat=N2, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
            <Style.Triggers>
                <Trigger Property="IsKeyboardFocusWithin" Value="True">
                    <Setter Property="Text" Value="{Binding SomeValue, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>

I think it isn't possible to change any properties of a binding after it has been applied. At least when i try to change the StringFormat property of an existing binding i get an InvalidOperationException saying Binding cannot be changed after it has been used. To me it seems that there is no way around creating a new binding every time the focus changes. — Clemens
Thought about this again, and considered some kind of attached Data property solution where the property change handler would attach GotFocus/LostFocus handlers etc. In the end i'd prefer to have a derived TextBox with properties Data and StringFormat, which would set the Text property according to their values and the current focus state. — Clemens

Jake Berger Jake Berger · Accepted Answer · 2012-02-11T06:13:51

Is there a way I can make this style generic, such as only changing the StringFormat property of the binding in the DataTrigger?

Inherit Style and new XAML would become this:

 <TextBox>
    <TextBox.Style>
        <local:FlyingStyle Binding="{Binding ElementName=This, Path=SomeValue}" StringFormat="F2" />
    </TextBox.Style>
 </TextBox>

Here's the class...

public class FlyingStyle : Style
{
    public FlyingStyle()
        : base(typeof(TextBox))
    { }

    string _stringFormat;
    public string StringFormat
    {
        get { return _stringFormat; }
        set
        {
            _stringFormat = value;
            CheckInitialize();
        }
    }
    Binding _binding;
    public Binding Binding
    {
        get { return _binding; }
        set
        {
            _binding = value;
            CheckInitialize();
        }
    }
    void CheckInitialize()
    {
        if (StringFormat == null || Binding == null) { return; }// need both

        Setters.Add(CreateSetter(Binding, StringFormat));

        var trigger = new Trigger
        {
            Property = UIElement.IsKeyboardFocusWithinProperty,
            Value = true,
        };
        trigger.Setters.Add(CreateSetter(Binding));
        Triggers.Add(trigger);
    }

    /// <summary>Creates the common <see cref="Setter"/>.</summary>
    static Setter CreateSetter(Binding binding, string stringFormat = null)
    {
        // must create a copy, because same binding ref but diff StringFormats
        var bindingCopy = new Binding
        {
            // these could be copies as well
            UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged,
            ValidatesOnDataErrors = true,
            Mode = BindingMode.TwoWay,
            Path = binding.Path,

            AsyncState = binding.AsyncState,
            BindingGroupName = binding.BindingGroupName,
            BindsDirectlyToSource = binding.BindsDirectlyToSource,
            Converter = binding.Converter,
            ConverterCulture = binding.ConverterCulture,
            ConverterParameter = binding.ConverterParameter,
            ElementName = binding.ElementName,
            FallbackValue = binding.FallbackValue,
            IsAsync = binding.IsAsync,
            NotifyOnSourceUpdated = binding.NotifyOnSourceUpdated,
            NotifyOnTargetUpdated = binding.NotifyOnTargetUpdated,
            NotifyOnValidationError = binding.NotifyOnValidationError,
            //StringFormat = set below...
            TargetNullValue = binding.TargetNullValue,
            UpdateSourceExceptionFilter = binding.UpdateSourceExceptionFilter,
            ValidatesOnExceptions = binding.ValidatesOnExceptions,
            XPath = binding.XPath,
            //ValidationRules = binding.ValidationRules
        };
        // mutex ElementName, so modify if needed
        // Source = binding.Source,
        // RelativeSource = binding.RelativeSource,

        if (stringFormat != null)
        {
            bindingCopy.StringFormat = stringFormat;
        }
        return new Setter(TextBox.TextProperty, bindingCopy);
    }
}

Note that my test was

the generic MainWindow
impl INotifyPropertyChanged
SomeValue INPC property
DataContext = this
x:Name = This

Can I change the properties of a binding in a DataTrigger, without knowing the binding itself?

5 Answers