title: Why should dictionary keys be immutable?
body: <p>Got a question about why they ask to use immutable objects as keys in a Dictionary.</p>
  
  <p>The question actually got in my head when I recently used a dictionary (not for the very purpose of a Hash table apparently though) to place Xml Node objects as keys. I then updated the nodes several times during the usage.</p>
  
  <p>So what does 'use immutable keys' really mean?</p>