Syntax error when using ? on end of property name in c# (.net 4.6 )
<p>What do I need to do to enable support for the new(ish) <code>?</code> at the end of variables/properties in c# ?</p>

<p>The following sample snippet :</p>

<pre><code>using System;
using System.Collections.Generic;
using System.Runtime.Serialization;

[DataContract]
public class Foo {

   [DataMember( Name = "available?" )]
   public bool available? { get; set; }

}
</code></pre>

<p>Produces the following syntax error in the code view :</p>

<p><a href="https://i.stack.imgur.com/4Z2Cg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4Z2Cg.png" alt="enter image description here"></a></p>

<h2>More information</h2>

<p>This started out as I copied the json data from the <strong>A request of "google.com" would return</strong> section on <a href="https://jsonwhois.com/docs" rel="nofollow noreferrer">jsonwhois.net/docs</a>, and pasted it into <a href="http://jsonutils.com/" rel="nofollow noreferrer">jsonutils.com</a> with the following options :</p>

<p><a href="https://i.stack.imgur.com/9GvU2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9GvU2.png" alt="jsonutils parameters"></a></p>

<p>Which output a nice DataContract for me, but some of the properties were using the <code>?</code> null/or qualifier, and the IDE doesn't seem to register them.  Is there a reference or something I need to add ?</p>