One of the benefits of writing an ASP.NET book is that it forces me to spend a lot of time spelunking deep in the bowels of ASP.NET uncovering all sorts of little gems I never noticed the first time around.

Many of these little morsels should end up in the book, but I thought I would blog about a few of them as I go along. 

This is all part of the weird situation I find myself in while writing this book. I thought I would just sit down and all the words would flow. Instead, no matter how motivated I am, everytime I sit down to write I spend two hours procrastinating for every one hour of writing.  What gives!?

In any case, one of the gems I discovered is the ClientScriptManager.RegisterExpandoAttribute method.  This method allows you to add custom properties to a control.  These properties are not rendered in the HTML as attributes, but simply attached to the control in the DOM via javascript.

This is nice for control authors who want to make a custom control client scriptable, but still maintain XHTML compliance, since XHTML compliance doesn’t allow arbitrary attributes for tags.

The following is a really simple example.  I present here a custom control that inherits from Label.

public class ExpandoControl : Label
    //Code to be filled in.

The AddAttributesToRender method is the appropriate place to call RegisterExpandoAttribute.

protected override 
    void AddAttributesToRender(HtmlTextWriter writer)
                , "contenteditable", "true");

Now we can access the contenteditable property of this control via client script. The following javascript demonstrates.

var expando = document.getElementById('expando');
alert('Content editable: ' + expando.contenteditable);

This is a good approach to take to develop a client-side api for your custom controls.