Cannot resolve IHttpHandler

For some reason when I am trying to create a class which implements IHttpHandler I cannot resolve IHttpHandler.

Statements like:

using System.Web开发者_开发问答;

are not helping either.

This is a class library project, I am following example here: http://www.15seconds.com/issue/020417.htm

What am I doing wrong?

Ensure that you are referencing System.Web from you project.