title: Visual Studio C#: Why is a "using" directive insufficient for some libraries?
body: <p><strong>Scenario:</strong> I needed to add HttpUtility to my project, and I started by adding "using System.Web" to my collection of using directives. However the HttpUtility class would still not resolve, and I discovered (via <a href="https://stackoverflow.com/questions/1366290/why-is-system-web-httputility-urlencode-giving-namespace-name-doesnt-exist-in-vi">this</a> question) that I needed to add a reference to my project.</p>
  
  <p><strong>Question:</strong> Why do I need to add a reference to this library when for most other classes a "using" directive will suffice?</p>