The System.Web.UI.TagPrefixAttribute class defines an assembly-level attribute needed to specify tag prefix aliases for custom controls. This attribute is used by tools such as Microsoft Visual Studio to automatically generate a Register directive in the ASP.NET page where the custom controls are used.

This directive registers the tag prefix with a namespace. Moreover, it specifies the assembly where the custom control code implementation resides. With this directive in place, you can use custom controls in a Web page declaratively.

For more information about using attributes, see Extending Metadata Using Attributes.