title: C#: What does the [string] indexer of Dictionary return?
body: <p>What does the <code>[string]</code> indexer of <code>Dictionary</code> return when the key doesn't exist in the Dictionary?  I am new to C# and I can't seem to find a reference as good as the Javadocs.</p>
  
  <p>Do I get <code>null</code>, or do I get an exception?</p>