Silverlight 3 Behavior causing XAML error

A recent XAML error I received from a Silverlight Behavior had me going round in circles trying to find the cause for quite a while. I was getting an AG_E_PARSER_BAD_PROPERTY_VALUE in code similar to the following:

<canvas x:name="Blah">
   <i:Interaction.Behaviors>
      <myapp:SpecialBehavior Source="{Binding SomeProperty}" />
   </i:Interaction.Behaviors>
   ...
</canvas>

The error identified the myapp:SpecialBehavior line as the culprit but didn't give me any further information so I proceeded to try and debug the binding to see what was going wrong. This didn’t shed any light on the cause, the binding was being created fine – the error was occurring later on.

This had me stumped for a couple of hours – I even tried setting up Framework source stepping only to find that the Silverlight 3 symbols weren’t yet available. In the end I stumbled upon the answer by chance – looking at the Canvas class in Reflector I noticed that it didn’t inherit from Control, only FrameworkElement via Panel. A quick check of my Behavior code and I found this:

public class SpecialBehavior : Behavior<Control>

It was the Behavior itself that was invalid in the Interaction.Behaviors property due to the incompatible type parameter. I changed Control to FrameworkElement and everything started working fine.