title: f# System.Char.ToUpper
body: <p>I have an exercise that asks for a function that converts all characters of a string to uppercase using </p>
  
  <pre><code>System.Char.ToUpper
  </code></pre>
  
  <p>So first I changed the string to a char array and changed the array into a list of chars </p>
  
  <pre><code>let x = s.ToCharArray()
  List.ofArray x
  </code></pre>
  
  <p>Next, I thought I would use List.iter to iterate through my list and use the System.Char.ToUpper function on each character.</p>
  
  <pre><code>List.iter (fun z -&gt; (System.Char.ToUpper(z)))
  </code></pre>
  
  <p>This is not working however. I get an error 'The expression was supposed to have unit but here has char.' What am I doing wrong? Is it a flaw in logic or syntax?</p>