title: Square bracket syntax in function's parameter in C#?
body: <p>I'm learning ASP.NET and stumbled upon this method declaration:</p>
  
  <pre><code>public IQueryable&lt;Product&gt; GetProducts([QueryString("id")] int? categoryId) {.....}
  </code></pre>
  
  <p>The tutorial said <code>categoryId</code> will be equal to query string "id" (From URL, like &amp;id=5) but the question is what is <code>[QueryString("id")]</code> syntax called? Is this usable outside ASP.NET and what will the application of this be?</p>