The ListItem control has a property Attributes of type AttributeCollection. When the ListItem control is inside a RadioButtonList control or a DropDownList control, key-value pairs that you assign to the Attributes property of the ListItem control (either programmatically or declaratively) are not rendered.
To work around this problem in the RadioButtonList control, use a group of HtmlInputRadioButton controls. For example:
In the "Resolution" section of this article, the HtmlSelect control was used to show how to implement expando attributes as a workaround for using the DropDownList control. While both classes do make use of the ListItem class internally, they support expando attributes differently. The HtmlSelect control does support using expando attributes because its programming model is more HTML-centric. However, supporting expando attributes with the DropDownList control is more involved than just rendering the attributes themselves. For example, you cannot use data-binding to support attributes with the DropDownList control.