title: Returning nullable string types
body: <p>So I have something like this</p>
  
  <pre><code>public string? SessionValue(string key)
  {
      if (HttpContext.Current.Session[key].ToString() == null || HttpContext.Current.Session[key].ToString() == "")
          return null;
  
      return HttpContext.Current.Session[key].ToString();
  }
  </code></pre>
  
  <p>which doesn't compile.</p>
  
  <p>How do I return a nullable string type?</p>