title: "this" in function parameter
body: <p>Looking at some code examples for <code>HtmlHelpers</code>, and I see declarations that look like this:</p>
  <pre><code>public static string HelperName(this HtmlHelper htmlHelper, ...more regular params )
  </code></pre>
  <p>I can't remember seeing this type of construct anywhere else - can someone explain the purpose of the &quot;this&quot;?  I thought that by declaring something public static meant that the class did not need to be instantiated - so what is &quot;this&quot; in this case?</p>