title: How to make reference type nullable in C# 7.3?
body: <p>I can't upgrade Net for now. I want to achieve this <code>public SiteOptionModel? Site { get; set; }</code> like <code>bool?</code>. But it says I should upgrade .Net but I am afraid, upgrading .Net will destroy quite a big project. Is there any way to achieve this?</p>
  
  <pre><code>public bool IsVisibleInGrid { get; set; }
  public SiteOptionModel? Site { get; set; }
  public bool? IsDeleted { get; set; }
  
  </code></pre>
  
  <p><strong>LinQ query showing error not having SiteId</strong></p>
  
  <pre><code>from truck in database.Truck
     where truck.CarrierId == carrierId
           &amp;&amp; (truck.IsDeleted == null || truck.IsDeleted == false)
     orderby truck.Code
     select new CarrierDetailViewModel.TruckModel2
     {
         Id = truck.TruckId,
  
         Site = new CarrierDetailViewModel.SiteOptionModel{
              Id = (int)truck.SiteId,
              Name = truck.Site.Name,
              Code = truck.Site.Code
         }
     } ;
  
  </code></pre>
  
  <p><strong>Error</strong></p>
  
  <blockquote>
    <p>Severity  Code    Description Project File    Line    Suppression State
    Error   CS8370  Feature 'nullable reference types' is not available in C# 7.3. Please use language version 8.0 or greater.   </p>
  </blockquote>