Web Forms :: Dynamic User Control And Setting Properties?

I have a user control (i.e. ascx) that comprises of two related list boxes (i.e. once you change the selected item in one list the contents of the other list update). I'm adding this user control dynamically to a page on the click event of a button (i.e. using LoadControl).I wish to set the list box datasource properties, as well as the default selected item of the parent list from my parent page so I created public properties in my user control to accomodate this. I also created an event in my user control that notifes the parent page when the selected item changes in one of the list boxes so I can update the contents of the other list box.

I understand that I have to recreate the user control on every postback in order for it to function correctly but I'm not sure exactly how I should be doing it. I've shown below what I have currently - whilst it works the problem is that on postback I'm always creating the user control with all its default properties and then updating the properties that need to change(?). For example, when the choice in theparent list box changes the datasource of the list box is set twice (once in AddControl and then again in OnParentChanged)? As the contents of the list boxes originate in a database it means I'm making unnecessary calls to the database (unless I do some something like store each unique set of data in the ViewState).

I'd be very grateful if someone could review how I've done things and illustrate how perhaps I should be doing things. I'm not sure if the way I've done this is considered the 'correct' way. For example there may be a better way of doing things would avoid setting the properties twice? I'm a bit unsure of the way I've set the datasource properties of the list boxes and bound them. I wished to set the datasource from the parent page because I might have a second user control on my page that may also use the same datasource and I could reuse it (as opposed to each control making its own call to the database) so I'd like to keep this feature if possible.It would be great if posting code in reply that you use my example as a guide so I know what you're referring to.Here's the code for my user control:

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Here's the code for my containing page:

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