Question

SAMPLE CODE:

public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(String propertyName) 
{ 
    PropertyChangedEventHandler handler = PropertyChanged; 
    if (handler != null) 
    {
        handler(this, new PropertyChangedEventArgs(propertyName));
    }
}

VS:

public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(String propertyName) 
{ 
    if (PropertyChanged!= null) 
    {
        PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
}

Why is it that I always see people creating assigning PropertyChanged to a “handler” instead of just using it?

Answer 1

If you do it the simpler way, and a different thread removes the last handler from the event just inside your if, you’ll get a null reference. (delegates are immutable)

By making a handler temporary, you prevent this, since you only check the field once.

If the event will never be unsubscribed to from multiple threads, you don’t need the temporary.