What is this C# construct doing?

Question

I have seen on another post this and its confusing me...

public class MyClass : INotifyPropertyChanged
{
   public event PropertyChangedEventHandler PropertyChanged;  

   protected void NotifyPropertyChanged(String info)
   {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
   }  

   public string MyProperty
   {
       set
       {
           if (_myProperty != value)
           {
               _myProperty = value;
               NotifyPropertyChanged("MyProperty");
           }
       }
   }
}

---

MyClass myClass = new MyClass();

myClass.PropertyChanged += delegate(object sender, PropertyChangedEventArgs e)
{
     actual = e.PropertyName;
};

I'm wondering about the last few lines are doing to be honest, why would the user be assiging a delegate to an event? Would't they assign a method to it (as an event handler) or even an anonymous method as the event handler?

I thought that events were meant to encapsulate delegates.....?

Answer 1

You always subscribe to an event (or unsubscribe from it) using a delegate. Even if you do:

button.Click += HandleButtonClick;

that's equivalent to

button.Click += new EventHandler(HandleButtonClick);

When you say:

Would't they assign a method to it (as an event handler) or even an anonymous method as the event handler?

That's exactly what the last few lines of code do. That's what delegate (...) { ... } is.

I thought that events were meant to encapsulate delegates.....?

Events provide an implementation of the observer pattern, using delegates as the observers.

Answer 2

This syntax was introduced in C# 2.0 They are using an anonymous method here, rather than having to create an actual instance method of the class. It's generally considered cleaner.

In C# 3 and above, a Lambda expression could have been used as well.