To illustrate the mechanics of Attached Behaviors, Karim Hernandez decided to implement a working RadioButtonGroup.
I was recently asked by a customer, “Why doesn’t RadioButton.GroupName work when a single RadioButton is put in a DataTemplate within an ItemsControl?” I had no answer. When I tried it out myself, lo and behold, two checked RadioButtons were staring back at me. A quick search told me that others have encountered this issue, and none had found a solution based in markup.