title: Creating nullable class
body: <p>How do I create a nullable (<code>?</code>) class in C#? Like:</p>
  
  <pre><code>public class Hello1 {
    public int Property1 { get; set; }
  }
  
  public class Hello2 {
   Public Hello1? Property2 { get; set; } //Hello1 should work when it has "?"
  }
  </code></pre>
  
  <p>I want to make <code>Hello1</code> class to be able to take the form <code>Hello1?</code> if needed.</p>