Question

I need an event to fire when I change a property. When I run this code I get a Object reference not set to an instance of an object. What am I doing wrong? What is the correct way to instantiate an event, and fire it when a property is set?

public member:

public event System.EventHandler ClassChanged;

property set:

ClassChanged(this, EventArgs.Empty);

Answer 1

You need to verify the event handler it not null first:

if (ClassChanged != null)
    ClassChanged(this, EventArgs.Empty);

But in general, you may want to wrap this up into a helper method like so:

private void NotifyClassChanged() {
    if (ClassChanged != null)
        ClassChanged(this, EventArgs.Empty);
}

Or possibly implement INotifyPropertyChanged instead.