The Image.DescriptionUrl property specifies an HTML file that provides additional details for the image. The Image.DescriptionUrl property renders as the longdesc attribute for the image control. If the Image.DescriptionUrl property is not set, or is set to an empty string (""), the System.Web.UI.WebControls.Image control renders without the longdesc attribute.

Use the Image.DescriptionUrl property to supplement the Image.AlternateText property. For example, if you use an System.Web.UI.WebControls.Image control to display an image of the solar system, you can set the Image.AlternateText property to The Solar System, and then set the Image.DescriptionUrl property to an HTML page that further describes the image of planets orbiting the sun in the solar system.

The Image.DescriptionUrl property is optional, but it can enhance the accessibility of your Web page for text-only browsers and screen readers. Browsers can use the detailed text description to convey important concepts in place of, or in addition to, the visual image. Refer to your browser documentation for details on how it uses the descriptive content specified by the longdesc attribute.