title: Efficient way to generate a list of missing elements given another list?
body: <p>I have a list with the elements <code>[1,2,3,4,5]</code>.</p>
  
  <p>If I have another list that contains elements of the mainlist such as <code>[1,2]</code> or <code>[2,4,5]</code>, what's an efficient way to generate new lists with the missing numbers, so that:</p>
  
  <p><code>[1, 2]</code> utilizing <code>[1,2,3,4,5]</code> would give <code>[3, 4 ,5]</code></p>
  
  <p><code>[2, 4, 5]</code> utilizing <code>[1,2,3,4,5]</code> would give <code>[1, 3]</code></p>
  
  <p>I was thinking of using a nested for-loop to check, but I was wondering if there's a more efficient way or built-in function that can be used in C#. </p>