title: Convert an integer to a binary string with leading zeros
body: <p>I need to convert int to bin and with extra bits.</p>
  
  <pre><code>string aaa = Convert.ToString(3, 2);
  </code></pre>
  
  <p>it returns <code>11</code>, but I need <code>0011</code>, or <code>00000011</code>.</p>
  
  <p>How is it done?</p>