The ClientScriptManager.RegisterExpandoAttribute(string, string, string) method registers a name/value pair as a custom (expando) attribute on the specified System.Web.UI.Control. The expando attribute is set dynamically from JavaScript to preserve XHTML compatibility for the rendered control's markup. Quotes and backslashes in the custom (expando) attribute's values are escaped. If you do not want to escape quotes and backslashes, call the ClientScriptManager.RegisterExpandoAttribute(string, string, string, bool) overload method and set the encode parameter to false.

If the expando attribute is not found or the control to add the expando attribute to is not found, the client script is still emitted, but it will not affect the control.