title: Variable type ending with ?
body: <p>What does <code>?</code> mean:</p>
  
  <pre><code>public bool? Verbose { get; set; }
  </code></pre>
  
  <p>When applied to <code>string?</code>, there is an error:</p>
  
  <blockquote>
    <p>The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable' </p>
  </blockquote>