Why doesn't the INotifyPropertyChanged work?

I used a Multibinding to bind some properties and use the INotifyPropertyChanged interface to notify these properties'changes.But sadly,it seems that the INotifyPropertyChanged didn't work. The PropertyChangedEventHandler "PropertyChanged" was null all the time.

Questions:

A. Could you please tell me why the event is null?In my mind,there should be a default method for the event PropertyChangedEventHandler,am I wrong?(Resolved,thanks!)

B. Just like some friends said,I tried again without using the INotifyPropertyChanged.But the target's property's value seems to be not changed...

1. Properties

   public static readonly DependencyProperty LeftOffsetProperty = DependencyProperty.Register("LeftOffset", typeof(double), typeof(NetworkTaskLable), new FrameworkPropertyMetadata(0d, FrameworkPropertyMetadataOptions.AffectsRender));
   public static readonly DependencyProperty TopOffsetProperty = DependencyProperty.Register("TopOffset", typeof(double), typeof(NetworkTaskLable), new FrameworkPropertyMetadata(0d, FrameworkPropertyMetadataOptions.AffectsRender));
   
   public double LeftOffset
   {
       get { return (double)GetValue(LeftOffsetProperty); }
       set { SetValue(LeftOffsetProperty, value); }
   }
   public double TopOffset
   {
       get { return (double)GetValue(TopOffsetProperty); }
       set { SetValue(TopOffsetProperty, value); }
   }
   

2. Multibinding (It seems work well.By using the converter to calculate a location which is affected by "TopOffset" and "LeftOffset")

   var multibinding = new MultiBinding() { Converter = new BeginAndStartDateToLeftConverter_NetworkTaskLable(), ConverterParameter = NetworkView };
   multibinding.Bindings.Add(new Binding("Start"));
   multibinding.Bindings.Add(new Binding("StartDate") { Source = NetworkView });
   multibinding.Bindings.Add(new Binding("LeftOffset") { Source = this });
   MainCanvas.SetBinding(LeftProperty, multibi开发者_运维问答nding);
   

3. INotifyPropertyChanged

   public event PropertyChangedEventHandler PropertyChanged;
   public void CallPropertyChanged(string PropertyName)
   {
       if (PropertyChanged != null)//It seems to be null all the time!!!And the properties'changes were never notified!!!
           PropertyChanged(this, new PropertyChangedEventArgs(PropertyName));
   }
   

4. Notify the change

   SetValue(LeftOffsetProperty, moveAdorner.LeftOffset);
   CallPropertyChanged("LeftOffset");
   SetValue(TopOffsetProperty, moveAdorner.TopOffset);
   CallPropertyChanged("TopOffset");
   

You don't need to implement INotifyPropertyChanged for dependency properties. Binding will track changes of these properties automatically.

Set Mode=TwoWay anywhere in your binding ?

You need to override OnPropertyChanged and check to see if the property is the one you are looking for.

protected override void OnPropertyChanged(DependencyPropertyChangedEventArgs e)
{
    if (e.Property == MyProperty)
    {
        // do something
    } 
    base.OnPropertyChanged(e);   
}