title: What is MemoryCache.AddOrGetExisting for?
body: <p>The behaviour of <a href="http://msdn.microsoft.com/en-us/library/dd988741.aspx">MemoryCache.AddOrGetExisting</a> is described as:</p>
  
  <blockquote>
    <p>Adds a cache entry into the cache using the specified key and a value
    and an absolute expiration value.</p>
  </blockquote>
  
  <p>And that it returns:</p>
  
  <blockquote>
    <p>If a cache entry with the same key exists, the existing cache entry; otherwise, null.</p>
  </blockquote>
  
  <p>What is the purpose of a method with these semantics? What is an example of this?</p>