Improvements to Silverlight Multi-binding support

7/19/2009 6:28:27 PM #

Hi Stefan,

Nice one - good improvement. Did you also see the little amendment I added at the bottom of the page to allow binding of attached properties:

www.scottlogic.co.uk/.../#comment-2124

Regards,
Colin E.

Colin E.

7/19/2009 11:52:09 PM #

Cool - glad you included that.

Yes, I did wonder about hard-coding the System.Windows.Controls namespace, however it was a compromise (these things always are). To do it properly I think you would have to allow namespace prefixes - I have no idea if the XAML parser exposes the namespace prefix mappings for a document. So, the compromise was just to keep it simple!

Colin E.

Colin E.

12/7/2009 1:12:10 AM #

Thanks for the article.

I can't get the multibindings to work when using more than one source. The bindings do not seem to be updated when specifying a source using ElementName. I adapted the example (to something non-sensical) to illustrate what I want to do. When MyListBox.SelectedItem changes, the bindings aren't updated (breakpoints in the converter aren't encountered).


<TextBlock x:Name="Block" Foreground="White" FontSize="13"
           Margin="5,0,0,0">
    <local:BindingUtil.MultiBindings>
        <local:MultiBindings>
            <local:MultiBinding TargetProperty="Text" Converter="{StaticResource TitleConverter}">
                <Binding Path="Surname" />                            
                <Binding Path="SelectedItem" ElementName="MyListBox"/>
            </local:MultiBinding>
        </local:MultiBindings>
    </local:BindingUtil.MultiBindings>
</TextBlock>


Is this a limitation of the implementation or am I doing something else wrong?

Erik

12/11/2009 8:57:17 PM #

Recently came by this article, and must say that its great! However, I am having a slight problem, although it is obviously one of those "by design" things, but perhaps you'll be able to help me.

I am using the code to format data in a DataGridTemplateColumn in a DataGrid. I have one MultiBinding with two Bindings (so that means two BindingSlave's being created). Once the DataGrid is bound (for simplicity, lets say there's just one row), each of the BindingSlave's Value property changes, and as a result, UpdateConvertedValue is being called twice. However, the first time it is called, the second BindingSlave does not yet have a value (well, it does - the null that is specified in the DependencyProperty ValueProperty) - I dont know the proper terminology, but its basically because it has not yet had its Value assigned. This presents a problem for when writing the Converter - it must account for the cases when the second BindingSlave has no value yet. In essense, that amounts to lots of "==null" checking, which may not be correct in some cases, since it just might be true that the value is null.

For now (i.e. it is enough in my case), I changed the code as follows to work around this:
BindingSlave class:
Change

public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value", typeof(object), typeof(BindingSlave), new PropertyMetadata(null, OnValueChanged));

to

internal static object ValueProperty_NoValue = new object();
public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value", typeof(object), typeof(BindingSlave), new PropertyMetadata(ValueProperty_NoValue, OnValueChanged));


MultiBinding class:
Change the UpdateConvertedValue method as so:

    private void UpdateConvertedValue()
    {
      var values = new List<object>();
      foreach (BindingSlave slave in Children)
      {
        // if the BindingSlave doesnt have a value yet, dont calculate the ConvertedValue
        var o = slave.Value;
        if(BindingSlave.ValueProperty_NoValue==o)
          return;
        values.Add(o);
      }
      ConvertedValue = Converter.Convert(values.ToArray(), typeof(object), ConverterParameter, CultureInfo.CurrentCulture);
    }


What this does it make sure that the Converter is only called when all the BindingSlaves have had the Value set.
This works in my case, but my case is the OneTime binding mode with nothing special being done by the DataGrid, it just displays data. I'll be glad to see a "fix" for this being done by people with better knowledge than me in Silverlight

Regards,
Anton A.

Anton A.

2/18/2010 7:15:36 PM #

Silverlight multi-binding for WPF

Silverlight multi-binding for WPF

Stefan Olson's Blog

4/19/2010 2:05:18 AM #

In Silverligh 4 I get the following exception.

System.ArgumentException: Object of type 'System.Windows.Data.Binding' cannot be converted to type 'System.Collections.ObjectModel.ObservableCollection

Florian

4/21/2010 6:32:12 PM #

I solved it temporarally by using using the default ContentProperty 'Children' and in xaml using the <BindingSlave /> control instead of <Binding />.

Florian

8/19/2011 1:02:22 AM #

Hi Stefan,

    I created a modification to your update to Colin's multi-binding.  I added the ability to get a parameter on ConvertBack using a dependency property.  I had a case where this was necessary in order to have a dynamic Dictionary<string, string> that could be bound two-way.
    Please see this article: flyinghighsoftware.wordpress.com/.../ and let me know what you think.

Brian Collins