Question

For part of a fairly-complex WPF ToolTip, I’m attempting to use a MultiBinding to produce formatted text based on two properties. The problem is, the binding’s MultiConverter receives DependencyProperty.UnsetValue for each item in its values array.

The following works, using a single Binding:

<ToolTipService.ToolTip>
  <StackPanel>
    <TextBlock>
      <TextBlock.Text>
        <Binding Path="Amt" Converter="{StaticResource singleValueConverter}"/>
      </TextBlock.Text>
    </TextBlock>        
  </StackPanel>
</ToolTipService.ToolTip>

And so does this, using a MultiBinding with StringFormat:

<ToolTipService.ToolTip>
  <StackPanel>
    <TextBlock>
      <TextBlock.Text>
        <MultiBinding StringFormat='{0:C} in {1}'>
          <Binding Path="Amt"/>
          <Binding Path="Currency"/>
        </MultiBinding>
      </TextBlock.Text>
    </TextBlock>        
  </StackPanel>
</ToolTipService.ToolTip>

But a MultiBinding with a Converter does not:

<ToolTipService.ToolTip>
  <StackPanel>
    <TextBlock>
      <TextBlock.Text>
        <MultiBinding Converter="{StaticResource multiValueConverter}">
          <Binding Path="Amt"/>
          <Binding Path="Currency"/>
        </MultiBinding>
      </TextBlock.Text>
    </TextBlock>        
  </StackPanel>
</ToolTipService.ToolTip>

The bindings in the last example don’t receive any value. This isn’t the case outside of a ToolTip – what is going on such that binding fails in this specific case?

Answer 1

Try setting Mode=”OneWay” on your binding.

Also, have you checked this problem and solution:
http://social.msdn.microsoft.com/Forums/en-IE/wpf/thread/15ada9c7-f781-42c5-be43-d07eb1f90ed4

The reason of this error is the
tooltips have not been loaded, so
DependencyProperty.GetValue returns
DependencyProperty.UnsetValue. You
should add some code to test that is
value is Dependency.UnsetValue. The
following code shows how to do this.

public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
    if (values[0] == DependencyProperty.UnsetValue || values[1] == DependencyProperty.UnsetValue) 
        return "";
    [...]
}