The expando attribute

By coincidence I noticed a method I’ve never seen before on the ClientScript property of the page class in ASP.NET 2.0. It’s called something as cryptic as RegisterExpandoAttribute and it’s very useful.

It can set JavaScript properties on any element you would normally reference with document.getElementById(‘elementID’). That means you can control the state of your HTML elements from the code-behind in a very easy manor. It also means you can control HTML elements that don’t have a runat=”server” attribute.

Here are two examples – one where a property is set on a server-control and one on a HTML element otherwise invisible to the code-behind.

And this is the JavaScript it produces:

<script type="text/javascript">
<!--
var txtPassword = document.all ? document.all["txtPassword"] : document.getElementById("txtPassword");
txtPassword.value = "britney";
// -->
</script>

I think where this really rocks the most is the ability to control regular HTML elements that hasn’t got the runat=”server” attribute. Those elements have always been invisible to the code-behind and now you have direct server-side access to their properties. The method doesn't give you anything you couldn't do before, but it makes it so much easier and cleaner.