How does WPF INotifyPropertyChanged work?

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How does this call activates ? What’s the C#’s magic behind
this to make it possible?

This code creates a Binding object which links the TextBlock’s Text property to the ViewModel property. It also adds an event handler to the ViewModel’s PropertyChanged event to update the text value when the ViewModel fires the PropertyChanged event (with the right property).

Why is if (PropertyChanged != null) necessary? Who sets up the
PropertyChanged to what value?

If the PropertyChanged event is null, then firing it will cause a NullReferenceException.

The meaning of Mode=TwoWay looks like that it not only can signal the
change, but also updates the content when other Binding element with
the same name in binding changes, then what about OneWay mode? Can we
set a Binding as source only or target only?

The binding modes are:

  • TwoWay: Changes the bound value when the ViewModel property changes and vice versa
  • OneWay: Changes the bound value when the ViewModel property changes only
  • OneWayToSource: Changes the ViewModel property when the bound value changes only
  • OneTime: Sets the bound value to the value of the ViewModel property just when the application is created or the data context changes.

You can read more about them here: http://msdn.microsoft.com/en-us/library/system.windows.data.bindingmode.aspx

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