title: Format a number with commas, but keep decimals
body: <p>I'd like to group the digits in a double by thousands, but also output however number of decimals are actually in the number.  I cannot figure out the format string.  </p>
  
  <pre class="lang-cs prettyprint-override"><code> 1000 =&gt; 1,000
   100000 =&gt; 100,000
   123.456 =&gt; 123.456
   100000.21 =&gt; 100,000.21
   100200.123456 =&gt; 100,200.123456
  </code></pre>
  
  <p>Disclaimers (it's not as straight forward as you think): </p>
  
  <ul>
  <li>Please do not point me to <a href="https://stackoverflow.com/questions/105770/net-stringformat-to-add-commas-in-thousands-place-for-a-number">this question</a> it does not answer my question</li>
  <li>Please do not point me to <a href="http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx" rel="nofollow noreferrer">MS</a><a href="http://msdn.microsoft.com/en-us/library/0c899ak8.aspx#" rel="nofollow noreferrer">DN</a>, the articles have not helped me.  (If you believe I'm reading them incorrectly, read them yourself and post the correct answer)</li>
  <li><code>.ToString("n")</code> does not work, it rounds the digits</li>
  </ul>