title: How to restrict a WebApi route parameter based on an array of possible values
body: <p>I am building a WebApi v.2 method for the following route:</p>
  
  <pre><code>[Route("api/{entityName}/logo/{entityId}/{size:char}")]
  public HttpResponseMessage GetLogo(string entityName, string entityId, char size) {...}
  </code></pre>
  
  <p>Now, I need to restrict <code>size</code> parameter to one of three characters: <code>s</code>, <code>m</code>, <code>l</code>. If <code>size</code> is not one of those, I don't want this route to be matched.</p>
  
  <p>Is it possible?</p>