title: Does adding virtual to a C# method may break legacy clients?
body: <p>The question is very straightforward,</p>
  
  <p>if I have a following class:</p>
  
  <pre><code>public class ExportReservationsToFtpRequestOld 
  {
      public int A { get; set; }
      public long B { get; set; }
  }
  </code></pre>
  
  <p>and change it to:</p>
  
  <pre><code>public class ExportReservationsToFtpRequestOld 
  {
      public virtual int A { get; set; }
      public virtual long B { get; set; }
  }
  </code></pre>
  
  <p>can it break a legay client dll?</p>