Monday, September 6, 2010

silent bug if you forget to implement INotifyPropertyChanged

public int PortNumber{
  get{
    return _portNumber;
  }
  set{
    if (_portNumber != value){
      _portNumber = value;
      OnPropertyChanged("PortNumber");
    }
  }
}
If host class Agent implements INotifyPropertyChanged, then the wiring works as follows
#1) users click a "++" button
#4) event handler increments PortNumber and saves it via the setter
#6) setter fires prop changed event
#9) xaml binding system queries PortNumber property via getter -- you can see this in a debugger

Now things break if you don't implement INPC. Xaml binding doesn't call the getter (#9) and won't display new value, even though
- event firing still happens (#6),
- xaml won't complain either

I think xaml will still bind to this Agent object, but won't treat Agent as a "Notifying source", so the binding system will ignore the event -- the invitation to re-query the VM for updates.

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