title: Why can't I .add a Dictionary(Key, Value) in a foreach?
body: <p>If I want to loop over a Dictionary(Key, Value)... why cant I add a new key-value pair in the loop?</p>
  
  <pre><code>Dictionary&lt;string, string&gt; persons = new Dictionary&lt;string, string&gt;();
  
  persons.Add("roger", "12");
  persons.Add("mary", "13");
  
  foreach (KeyValuePair&lt;string,string&gt; person in persons)
  {
   Console.WriteLine("Name: " + person.Key + ", Age: " + person.Value);
   persons.Add("Xico", "22");
  }
  </code></pre>