How can I bind against two DynamicResources? - wpf

Question

I am trying to bind the value of an element to a first resource , if present, then another one otherwise.
In other words, if the resources look like
<s:String x:Key="first">Hello<s:String>
<s:String x:Key="second">World<s:String>
my element's value would hold Hello
But if the resources have only
<s:String x:Key="second">World<s:String>
the value would be World
I have tried a number of solution but none seems to work or is elegant enough.
I wish I could write
<MyElement>
<MyElement.Value><MultiBinding Converter=...><DynamicResource Key=First/><DynamicResource Key=Second/> ...
where the converter takes care of finding the first non null value.
However, WPF does not allow mixing DynamicResource and MultiBinding
Do you have a solution?

Answer 1

Edit 1: I may have read your question a little too fast... you're binding to dynamic resources... not class properties... so the solution below is probably not what your looking for. But I'll leave it for now in case it helps you come up a solution.
Edit 2: I tried the following code in Visual Studio 2010 SP1 and it works like it should in the designer (commentout/uncomment the resource 'first'). But it fails to successfully build... which I find strange:
<TextBlock>
<TextBlock.Resources>
<!--<s:String x:Key="first">Hello</s:String>-->
<s:String x:Key="second">World</s:String>
<l:NullItemConverter x:Key="NullItemConverter" />
</TextBlock.Resources>
<TextBlock.Text>
<MultiBinding Converter="{StaticResource NullItemConverter}">
<Binding Source="{DynamicResource first}" />
<Binding Source="{DynamicResource second}"/>
</MultiBinding>
</TextBlock.Text>
</TextBlock>
public class NullItemConverter : IMultiValueConverter
{
object IMultiValueConverter.Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return values[0] ?? values[1];
}
object[] IMultiValueConverter.ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
Original Answer (that doesn't answer your question but may help depending on your situation):
Assuming your two properties are in the same class, could you make a third property that smartly outputs the correct value and bind to that instead:
public class MyObject : INotifyPropertyChanged
{
private string property1;
public string Property1
{
get { return this.property1; }
set
{
if (this.property1 != value)
{
this.property1 = value;
NotifyPropertyChanged("Property1");
NotifyPropertyChanged("PropertyForBinding");
}
}
}
private string property2;
public string Property2
{
get { return this.property2; }
set
{
if (this.property2 != value)
{
this.property2 = value;
NotifyPropertyChanged("Property2");
NotifyPropertyChanged("PropertyForBinding");
}
}
}
public string PropertyForBinding
{
get
{
return this.Property1 ?? this.Property2;
}
}
public MyObject() { }
#region -- INotifyPropertyChanged Contract --
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String info)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
#endregion INotifyPropertyChanged Contract
}