This article is part of a series of posts about various aspects of writing web controls for ASP.Net using an ad rotator as an example. The AdRotator WebControl Example post has links to related posts and downloads.

The AdRotator needs to insert a class definition script in the output once, even if multiple AdRotator controls are declared on the page. It also needs to insert a script that instantiates that class prototype for every AdRotator control on the page. Fortunately there are static methods on the System.Web.UI.Page object that will do this for us.

Page.ClientScript.RegisterStartupScript(Type type, string key, string script)

The keys here a crucial, there is only one script rendered for each key. If we inject the definition script with a static key it means no matter how many instances control try to register it, only one instance of script will be sent to the output.

To to insert a unique script for every instance of the AdRotator we need to use a unique key. I use the server side controls ClientId property which will be difference for every instance of the control on any page. You could also use a GUID.

This is clearly the weakest post of all the posts about the AdRotator. I just kind of got this working an never really got very inspired to learn anything else about it. I posted it anyway as I felt I did need to cover this topic for completeness of the series. I'll try to update it soon.