title: Verifying that a string contains only letters in C#
body: <p>I have an input string and I want to verify that it contains:</p>
  
  <ul>
  <li>Only letters or</li>
  <li>Only letters and numbers or</li>
  <li>Only letters, numbers or underscore </li>
  </ul>
  
  <p>To clarify, I have 3 different cases in the code, each calling for different validation. What's the simplest way to achieve this in C#?</p>