c# - Why is ImageSource called twice in WPF? -

my app changes pictures button click. however, when watch breakpoint, imagesource's property referenced twice each time. other properties referenced once.

why imagesource "get" running twice?

specifically, line runs 2 times, back, right after propertychanged():
get { return picture; }

this how update image in gui:

public class applicationstate : inotifypropertychanged {     public event propertychangedeventhandler propertychanged;      private imagesource picture;      public imagesource picture     {         { return picture; }         set         {             picture = value;             notifypropertychanged("picture");         }     }      private void notifypropertychanged(string propertyname)     {         if (propertychanged != null)         {             propertychanged(this, new propertychangedeventargs(propertyname));         }     } } 

this xaml code image:
<image source="{binding path=picture}"/>

and picture change triggered with:
picture = convert(picturepath);

am missing something? why ever run twice back?

edit: microsoft explains of reason in this note.