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Don Makoviney
Don Makoviney

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Effective interaction between user controls and parent container

In this article I'll cover topics related to the interaction between user controls and parent container (parent page).
In the classic ASP, when we had similar layout elements for a set of pages, we were forced to use dynamic includes. This is quite a good solution, because it allows to get rid of repeating code, but, nevertheless, there are some flaws in it. Just for example, there were many problems related to virtual paths in those include files - one had to keep track of all such entries in the include files, because it could cause problems after deploying to a web site with another configuration.

Moreover, when we speak about .NET world, we should keep in mind one of the key benefits of ASP.NET platform - code separation from implementation layer. This means that if we want to keep purity of this concept, we have to get rid of dynamic includes and replace them with a user controls.

So, how user control can interact with the parent page class which it declared in? Of course, it can expose some properties, but this is what I call "static interaction" - you just can set or get some of the user control's properties. More interesting thing is a "dynamic interaction", when some of the user control events could be handled in parent page class. This is also called "event bubbling".
So here you can see short and simple example of it. User control contains dropdown list and event handler for it. This event handled not only in the user control, but in the page class as well.

Let's take a look on what's happening. When user changes selection of this dropdown list, and makes new request therethrough, first of all Page_Load() method of parent page container is fired. At this moment user control is loaded and then Page_Load() method of this control class is fired. Then control is passed to the event handler in the user control, and, finally event is "bubbled" back to the parent page class and special event handler for this event is fired there. So, that's the sequence of called methods. This allows to execute some code in the parent page class after user control rendering, what is very useful option.

Here you can see user control code:

Ok, here're code comments:
First of all we have to declare delegate for handling our event in the parent page class. This is done by the following statement:

public event EventHandler SelectedIndexChanged;

Then in the event handler for OnSelectedIndexChanged event (Customer_OnSelectedIndexChanged(object sender, EventArgs e) function), we fire this delegate by the following statement (first check if the certain event handler was declared on the parent page):

if ( SelectedIndexChanged != null)
{
SelectedIndexChanged(this,e);
}

Also do not forget to set AutoPostBack property to true. Otherwise form will not be submitted to a server after selection been changed.
Here you can see parent page code:

Parent page code is very simple. Only thing we have to do is to declare event handler (OnActiveCustomerChanged(object sender, EventArgs e) function).
Mind that this event handler's signature is not arbitrary and must follow EventHandler delegate's signature.