I would like the user to change the symbology during runtime just like the Symbol Gallery. Meaning loading a ListBox of ResourceDictionary items(in a separate resource dictionary xaml) which allows the user to choose the desired symbology he wants...
Is there a open source code for the Symbol Gallery?
Unfortunately, there is no plan for having the Symbol Gallery as an open source resource/module at this time however; the structure of ESRI's interactive SDK is similar and it's available for you to download.