title: How to mock/ override readonly fields for unit testing
body: <p>I am using nj4x and it has a class that has readonly fields as follows.</p>
  
  <pre><code>   public class MarketInformation
      {
      public readonly double ASK;
      public readonly double BID;
      public readonly double DIGITS;
      public readonly double EXPIRATION;
      public readonly double FREEZELEVEL;
      }
  </code></pre>
  
  <p>I am writing unit tests for classes and methods writing these classes. For isolation/ mocking I am using Nsubstitute and MicrosoftFakes (shims mainly). I want to override these fields so when they are called in my methods I get predictable results for testing.
  I generated shim for this class but all it provides me is a constructor, now allowing me to initialize it still. Is there any way to set these fields from outside this class?</p>