title: The annotation for nullable reference types should only be used in code within a '#nullable' context
body: <p>I have a console app to try out the C# 8 null reference types. Switched the project to build with lang ver C# 8.</p>
  <p>Then the following code results in a warning.</p>
  <pre><code> class Program
      {
          static void Main(string[] args)
          {
              string? message = &quot;Hello World&quot;;
              string message2 = null;
  
              Console.WriteLine(message);
              Console.WriteLine(message2);
  
              // The annotation for nullable reference types should only be used in code within a '#nullable' context
          }
      }
  </code></pre>
  <p>What does this actually mean?</p>