title: How do I use ranges in case statement c#
body: <p>I want to know what I am doing wrong here:</p>
  
  <pre><code>string grade;
  switch (percentageMark)
  {
      case (90 - 100):
          grade = "A*";
          break;
      case (80 - 89):
          grade = "A";
          break;
      case (70 - 79):
          grade = "B";
          break;
      case (60 - 69):
          grade = "C";
          break;
      case (50 - 59):
          grade = "D";
          break;
      case (40 - 49):
          grade = "E";
          break;
      case (0 - 39):
          grade = "U";
          break;
  }
  </code></pre>
  
  <p>I get errors saying that "The label 'case -9:' already occurs in this switch statement", but only at cases 3, 4, 5 and 6. Could someone tell me why I am getting these errors</p>