Understanding the guts of Model-View-ViewModel

I am new to Model-View-ViewModel and wonder if the .NET Framework triggers whether a property has changed it's value or not.

I see that inside the Set of properties, the

                base.OnPropertyChanged("IsSelected"); (for example)

is called.  If the value did not change, does the .NET Framework then ignore this event?

What confuses me is this line of code:

            PropertyChangedEventHandler handler = this.PropertyChanged;
            if (handler != null)

What ever makes PropertyChanged not null?

This part, if done by the .NET Framework, I can then understand.  Otherwise I do not see in the demo program where this is done...

Here is the demo I am using:
http://msdn.microsoft.com/en-us/magazine/dd419663.aspx


Thanks,
newbieweb

/// </summary>
        public event PropertyChangedEventHandler PropertyChanged;

        /// <summary>
        /// Raises this object's PropertyChanged event.
        /// </summary>
        /// <param name="propertyName">The property that has a new value.</param>
        protected virtual void OnPropertyChanged(string propertyName)
        {
            this.VerifyPropertyName(propertyName);

            PropertyChangedEventHandler handler = this.PropertyChanged;
            if (handler != null)
            {
                var e = new PropertyChangedEventArgs(propertyName);
                handler(this, e);
            }
        }

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