title: Trying to validate c# program for alphabet. It pops up various errors. I am unable to find where I am wrong
body: <blockquote>
    <p>Error: "Too many characters  in character literal."</p>
  </blockquote>
  
  <p>If we replace XL with X,it pops up a error saying </p>
  
  <blockquote>
    <p>"Operator '||' cannot be applied to operands of type 'int' and
    'bool'"</p>
  </blockquote>
  
  <p>Any suggestions to make it work? I appreciate your valuable time.</p>
  
  <pre><code>Console.WriteLine("plan: ");
  
  string plan = Console.ReadLine();
  
  if (plan.Length = 0 || plan.ToUpper()[0] == 'S' || 
      plan.ToUpper()[0] == 'M' || plan.ToUpper()[0] == 'L' || 
      plan.ToUpper()[0] == 'XL')
  {
       Console.WriteLine("Invalid");
  }
  </code></pre>