title: checking if parameter is one of 3 values with fluent validation
body: <p>I have a class containing one string property:</p>
  
  <pre><code>public class Bla
  {
      public string Parameter { get; set; }
  }
  </code></pre>
  
  <p>I would like to write a custom AbstractValidator, which checks that Parameter is equal to either one of these strings:</p>
  
  <pre><code>str1, str2, str3
  </code></pre>
  
  <p>I guess this would be a starting point:</p>
  
  <pre><code>RuleFor(x =&gt; x.Parameter).Must(x =&gt; x.Equals("str1") || x.Equals("str2") || x.Equals("str3")).WithMessage("Please only use: str1, str2, str3");
  </code></pre>
  
  <p>but can I chain this and also show an error message, ideally without hard-coding the possibilities, e.g.:</p>
  
  <pre><code>Please only use: str1, str2, str3
  </code></pre>