A delegate declaration is just a template which tells the system which methods (by return type and number/type of parameters) can be called via an object of the delegate type. As such the delegate has no body and so you can't include any code in the declaration.

An event is just a delegate which only allows certain operations to be done to it from outside the class in which it is defined. Those operations are adding or removing eventhandler methods using (from C#) the += and -= operators respectively. When the event is fired (by calling the delegate), the eventhandlers are all called in the order they were added to its invocation list.

If you want to execute some code each time the event fires, then I'd do that in the method which is called to fire it:

public delegate void DeviceDisconnectedHandler();

public class MyClass
{   
    public event DeviceDisconnectedHandler DeviceDisconnected;
    
    public void OnDeviceDisconnected()
    {
        if (DeviceConnected != null)
        {
           // execute some code
           // now fire event
           DeviceConnected();
        }
    }
     
     // other members
}

There is a pattern that should always be followed when defining your own events.

1. Create an event args class that inherits from EventArgs (even if it's empty)


2. Utilize the generic EventHandler<T> delegate (saves you from having to define your own delegate)


3. Add the event to your class
   

   public CustomClass {
       public event EventHandler<CustomEventArgs> CustomEvent;
   }



4. On classes that are not sealed, provide a protected virtual method named On<EventName> that accepts as a parameter your custom event args


5. Inside this method implement whatever event specific code is needed for your class, but remember if an inheritor overrides this method and doesn't call base.OnEventName that the event won't get fired and your custom logic won't get invoked
   

   public CustomClass {
       public event EventHandler<CustomEventArgs> CustomEvent;
   
       protected virtual void OnCustomEvent( CustomEventArgs e) {
           // before event logic here
           if( CustomEvent != null ) CustomEvent( this, e );
           // after event logic here
       }
   }



6. Everywhere that you fire this event, use the OnEventName method rather than calling the event directly.