Declaration of event accessors.

In C# event accessors are used to add and remove event handlers in client code. By declaring custom accessors you can change subscription behavior.

Problem: Prevent a client object from subscribing to the same event multiple times.

Example: You have a view that contains a Navigate event which is raised when the user needs to navigate to the next page.

class View : IView
{
  public event NavigationHandler Navigate;

  ...

  public void raiseNavigate()
  {
    Navigate(typeof(View2));
  }
}

When Navigate is raised a presenter should clear the existing view and present a new one.

class FormPresenter
{
  ... constructor inject a view factory ...

  public void NavigateTo(Type type)
  {
    IView view = viewFactory.Find(type);
    view.Navigate+=new NavigationHandler(NavigateTo);
    changeView(view);
  }
}

This code works well if a new view is returned by the viewFactory every time. However, if the same view is returned from the factory, the presenter will subscribe to the Navigate event multiple times.

Solution: Subscribe to the Navigate event only if the presenter hasn't previously. Declaring the event accessors allows us to manage a single Navigate subscription per client model in the publisher.

Thus, View becomes:

class View : IView
{
  private NavigationHandler navigationHandler;

  private ArrayList navigateSubscriberInstances = new ArrayList();

  public event NavigationHandler Navigate
  {
    add
    {
      if (navigationHandler==null || !navigateSubscriberInstances.Contains(navigationHandler.Target))
      {
        navigationHandler += value;
        navigateSubscriberInstances.Add(navigationHandler.Target);
      }
    }
    remove
    {
      navigationHandler -= value;
      navigateSubscriberInstances.Remove(navigationHandler.Target);
    }
  }

  protected void raiseNavigate(Type type)
  {
    if (navigationHandler!=null)
    {
      navigationHandler(type);
    }
  }
}