title: String.Empty in Switch/case statement generate a compiler error
body: <p>If <code>String.Empty</code> is as good as <code>""</code>, then how come the compiler throws up with string.Empty in the <code>case</code> statement? Nothing can be more constant than <code>string.Empty</code> in my view. Anyone know?  Thanks! </p>
  
  <pre><code>switch (filter)
              {
       case string.Empty:  // Compiler error "A constant value is expected"
  
                  break;
  
                  case "":  // It's Okay.
                      break;
  
              }
  </code></pre>