title: string.IsNullOrEmpty returns true when supplied string is not null
body: <p>I have a unit test that calls a method on an object passing in a string.</p>
  
  <p>One of the first things that the method being called does is to check the string for null or empty.  </p>
  
  <p>However, no matter what the value of filePath is, the call to string.IsNullOrEmpty is true.  See image below.</p>
  
  <p><img src="https://i.stack.imgur.com/WwkSD.png" alt="Why is string null or empty"></p>
  
  <p>Am I missing something here? </p>
  
  <p><strong>EDIT:</strong>  </p>
  
  <p>Checking for null and string.Empty separately works as expected:</p>
  
  <p><img src="https://i.stack.imgur.com/oYWRL.png" alt="enter image description here"></p>
  
  <p><strong>EDIT 2:</strong></p>
  
  <p>I have cleaned the solution, deleted the bin directory via the file system and still, after a rebuild, the debugger shows that the ArgumentNullException should be being thrown, although it <strong>actually is not being thrown</strong>.</p>