title: Why concatenating two null string results an empty string?
body: <p>I just saw a weird result while I tried to concatenate two <code>null</code> strings: it returns an empty one! I can not imagine if it have some utility or why this occurs.</p>
  
  <p>Example:</p>
  
  <pre><code>string sns = null;
  sns = sns + sns;
  // It results in a String.Empty
  
  string snss = null;
  snss = String.Concat(snss, snss);
  // It results in a String.Empty too!
  </code></pre>
  
  <p>Can someone tell me why it returns a <code>String.Empty</code> instead of <code>null</code>?</p>