title: Check if input is number and only contains digists 0-9
body: <p>I'm using C#.
  How would I verify whether an input string is an integer and is only made up of digits 0-9?</p>
  
  <p>An additional requirement is that it should be always made up of exacly 9 digits total; no more, no less.</p>
  
  <p>e.g.,</p>
  
  <blockquote>
    <p>OK: 118356737, 111111111, 123423141,</p>
    
    <p>ERROR: 11a334356,
    1.2.4.535,
    1112234.222 etc</p>
  </blockquote>
  
  <p>thanks</p>