AddDays() not working within a while loop
<p>Is there anything that stops the <code>DateTime AddDays()</code> method that doesn't run within a while loop. I have this simple bit of code;</p>

<pre><code>DateTime last_day = monthCalendar2.SelectionRange.End;
DateTime first_day = new DateTime(year, month, day);

//Insert dates into vector
while (first_day != last_day)
{
  dates.Add(first_day);
  first_day.AddDays(1);
}
</code></pre>

<p>I step through the program and first_day never changes, anyone know why?!</p>
