title: Silverlight 4 Binding to ConverterParameter
body: <p>I have a ValueConverter which needs to be called with a dynamic parameter, depending on a property. I can't see a way to do this ...</p>
  
  <pre><code>Width="{Binding ActualWidthValue, Source={StaticResource VisibleSize}, Converter={StaticResource Fraction}}"
  </code></pre>
  
  <p>The "Fraction" converter get's (or should get) a parameter of type System.Size, which contains a numerator and denumerator. This value (should) depend on a ItemCollection.Count. Resetting the ItemCollection should reinvoke the Converter with the new values.</p>
  
  <p>My first idea was to manually change the ConverterParameter in CodeBehind on the PropertyChanged event of my ItemCollection DependencyProperty. But, as I know now, Silverlight has no GetBinding() method.
  I heard about GetBindingExpression and tried to do. But MyGrid.GetBindingExpression(Grid.ActualHeightProperty) is always returning null, although the Binding is already established.</p>
  
  <p>So, what can I do to reach my target? </p>
  
  <hr>
  
  <p>My implementation was not much different. I set the ConverterParameter in CodeBehind just before the Converter is called via Binding. That hasn't worked (Parameter contains still the initialization value).</p>
  
  <p>I'll try to use your suggestion. But why ConverterParameter can't be a DependencyPropery. What's the idea behind this? Does anybody know?</p>